The other answers cover most of what is being asked here but I thought I should add that a major factor in what caused these inabilities was a lack of economies of scale. Whereas the Romans had highly organised production, distribution and retail of both consumables and materials, the Feudal era was highly decentralised. The Feudal era fiefs had only small scale local production, in both agriculture and materiel. Water was another major issue, water quality was poor, and cavalry especially consumed a lot of it. Whilst a lot of the water infrastructure put in place by the Romans remained but it's distribution was not nearly as effective as it is today. Agricultural techniques were primitive so irrigation required a lot of water in order to maintain even meagre crop yields.
Think of it this way: Is it more effective to have 1000 local blacksmiths managing not only the production of war materiel but also meeting requirement for producing local goods (say horseshoes, ploughs and the like); than to have 10 forges dedicated to producing only swords, shields, horse bits - etc. etc. ? Both a centralised government (e.g the Romans) and a Feudal lord may only be able to dedicate 10% of surplus production to a war effort but the 10% surplus of a centralised government is far in excess of the 10% of 100 fiefs.
This, as mentioned by others, is one reason why sovereigns could not support large armies, nor support them for very long. Most armies, even in the Roman era, often had to survive by foraging off the land, and this was only really achievable in the warmer months of the year. Texts like Livy and Pliny make repeated reference to the campaign season, which one would interpret as the spring and summer months.
This leads into another issue - transportation. Because the vast majorities of troops were foot levies, and there were realistically only a few campaigning months in a year, the time required for a levie to arrive in time for campaigning was very limited. Lines of communication were not only long, but they were poor, with messages being transferred via courier on horseback. This meant mobilisation took time. As the Romans not only in later years had professional standing armies but a strong martial culture throughout, mobilisation was efficient.
Troops travelling in the winter would suffer high rates of attrition: through starvation, desertion and death from the elements. An army could not forage in the winter, as there were no harvests, and what stores were available were usually holed up in the local lords castle. Sieges were a dicey proposition at the best of times, and an army laying siege to access stores in the winter was in dire straits indeed. The further an army advanced the longer it's lines of supply grew and the more vulnerable they become. Commanders would have to garrison these lines of supply and this in turn would affect combat strength.
Perhaps a better question to think about is - what were the similarities between the Roman and the Colonial/Imperial age logistical systems that allowed them to field such large armies so effectively? Perhaps rather than focusing on why medieval armies fielded so few troops, consider what enabled the powers of antiquity and the Colonial to project force so effectively.
In the case of the Athenians, it was their naval power, in the case of the Romans it was their road networks; later with the Imperial powers it was again the ability to project power using navies and the widespread expansion of the railroad that made large scale warfare a cost-effective proposition.
In the 1700s, a number of technological revolutions took place that allowed for sovereigns to field large armies. First of all the rise of the Westphalian nation state allowed for central governance of large economies and professional militaries. With the decline of feudalism, governments were again able to achieve the kind of economies of scale that allowed them to dedicate significant surplus resources to war production - a trend that continued for some time and culminated in the Great War and World War 2.
The advent of the railroad allowed armies to be mobilised and deployed quickly. This was probably the single most important innovation in military history to date since the domestication of the horse, the development of the ship (is there a better word than ship? Naval transport perhaps?) and the prevalence of the Roman road network. It improved lines of communications and allowed a general to control forces over a greater geographical area.
Compare the levels of command and control. Fiefs were essentially little micronations often speaking different dialects; feudal lords under one liege may actually be rivals on their own turf. Being able to effectively dictate and command large armies requires that a general can communicate quickly and effectively with his troops. As rival fiefs may have their own agendas or may not wish to assist a competitor achieve glory or curry favour with the monarch, discipline varied widely. Consider the evolution of the hierarchies in modern militaries, their evolution from the early standing armies, and compare to them Roman military organisational reforms, like the Marius reforms. Then consider the flattened hierarchies of the medieval eras, and compare the performances across all three.
The majority of a feudal army would have been made up of peasant conscripts who were pressed into service with very little training and very little motivation to fight for a feudal lord who was likely a brutish taskmaster who profited off the backbreaking labour of his subjects. Whilst the weapons of the Imperial days did require steel and a good deal of it, on a per capita basis, the amount steel required to be effective was small, and the available production of steel was relatively high.
Compare this to the average heavy infantryman of the dark ages: His weapon was all steel and very likely to break in combat, his armour was steel or iron if he had any at all and the available production to support it was quite low. Apart from the richest lords, most soldiers and knights had to pay for their armour out of pocket, or seize it from the dead. There would have been little uniformity as all items were handcrafted, and only the richest soldiers could afford tailormade armour.
Keep in mind that even by the Napoleonic wars, the size of armies only really began to increase by the end of the 1700s. However from an organisational point of view, logistics and command and control hadn't begun to improve very much. Tt Austerlitz, the number of troops who actually fought were sixty and eighty thousand respectively between the Allies and the French.
Though the Armies fielded by Napoleon were as a proportion of population not considerably larger than those of a few centuries earlier, his disastrous invasion of Russia shows what happens to a large army when the infrastructure isn't there to support it.
After the pyrrhic victory at Poltava and Muscovy, the Russian winter decimated his Grand Armee. Without the ability to both supply and transport it, it fell apart from attrition and desertion. This demonstrates quite clearly that armies of a certain size require a base-line of logistical support.
Even as we look at the American Civil War we can see even with technological factors accounted for, the ability of the minds of the day to effectively control large armies of a million men upwards was very low. Incredibly high casualties, military blunders were all underscored by a lack of effective command and control and discipline. In World War I, it was quite obvious that commanding armies of such enormous sizes, technological and logistical capability was far beyond the ability of the commanders of the day. Only after four years of total war and immeasurable bloodshed were the commanders of the day able to develop the tactical and logistical solutions to solve the technical challenges they faced.
However, if we look at a number of the smaller campaigns in both the Civil War and World War I (for example The War in the West (Civil War) and the Palestine Campaign (WW1) we see that commanders were quite capable of managing their forces and pulling off quite technical battles of manoeuvre and combined arms long before these were integrated in army wide doctrines by the end of the the respective wars.
Only in the second world war did we see truly vast numbers of soldiers being controlled effectively by the Allies and the Germans.
From that high point now we've seen actually a great decline in the absolute size of armies to the point where today even the U.S combat forces only number around 350,000. It seems likely that there is an optimum size for an army beyond which command and control becomes difficult and the number of support personnel required to support it balloons out to unsustainable numbers.
- Economies of scale: Sovereigns of Feudal states could muster resources far less effectively due to the inherent inefficiencies in feudal economies due to a lack of economies of scale
- Logistics: Both the Roman Empire and the Colonial Empires of the 1700s understood how important transport networks were and invested heavily in them. In the case of the Romans it was roads, the European Imperials it was railways.
- Discipline: Both the Europeans and the Romans had professional standing armies who were motivated to fight and could expect a good reward for decent service.
- Command and Control: The Romans had centuries of practise in mobilising, deploying and fighting with large armies. The Europeans had less experience and were far less successful at doing so, but they had superior technological advantages such as the railroad and the telegraph that overcame many of the shortcomings.
- Social: The societies of both the Romans and the later Europeans were centralised nation states with strong social hierarchies that rewarded loyalty and punished insubordination (to an extent)
- Technological: The doctrines of the Middle Ages called for heavily armoured cavalry and infantry without the requisite levels of industry to support them. The Roman armour was mostly hardened leather, and they still used wood as a large component of a number of their weapons. The European armies did not requires the same amount of steel per soldier as the advent of gunpowder had made plate and chain mail obsolete.
- Environmental: The effects of the Black Death would've affected the available numbers of manpower reserves and surplus agricultural production required for a large scale war effort.