I was always under the impression that they managed to match and outdo Romans in terms of numbers most of the time.
They did to some degree, though it was in large part because of how they approached war. The Roman armies were professionals, men of a certain age that could enter the army for pay and glory.
The 'Barbarians' were not a professional army in this manner, they were not raising armies to march outwards, they were fighting for their very right to exist as people. As such, their armies were composed of the women and youth that would not be a part of the Roman army which saw their numbers rise significantly higher than the Romans numbers were. Many Roman conquests acknowledged such:
138 BC – The Roman, Sextus Junius Brutus found that in Lusitania the women were "fighting and perishing in company with the men with such bravery that they uttered no cry even in the midst of slaughter". He also noted that the Bracari women were "bearing arms with the men, who fought never turning, never showing their backs, or uttering a cry."
102 BC – A battle between Romans and the Teutonic Ambrones at Aquae Sextiae took place during this time. Plutarch described that "the fight had been no less fierce with the women than with the men themselves... the women charged with swords and axes and fell upon their opponents uttering a hideous outcry."
102/101 BC – General Marius of the Romans fought the Teutonic Cimbrians. Cimbrian women accompanied their men into war, created a line in battle with their wagons and fought with poles and lances, as well as staves, stones, and swords. When the Cimbrian women saw that defeat was imminent, they killed their children and committed suicide rather than be taken as captives.
There are quite a few more examples out there, especially if you get out into the Scythian peoples that gave rise to the legendary Amazon warriors (their women were actively buried alongside their weapons of choice). It should be noted that these Barbarians 'pantheons' frequently included warrior women and gave status to women as equals to men on the battlefield. Germanic women were frequently on the battlefield (either as active combatants or cheerleaders), which the Romans found extremely distasteful.
It's likely the Romans did play up the number game stating their warriors were victorious despite the numbers/odds. That being said, when it's your professional army (men of a certain age) vs the entire population they are slaughtering, then yes...that alone implies they were heavily outnumbered at times.
Boudica is an interesting story...thought to be leading over 100k men and women (including accounts of women in her army outnumbering men), and possibly as high as 230k soldiers (though thats from a source known to exaggerate). Roman victory was in good part due to their professional nature...chain of command and discipline allowed them to effectively field a large army while their opponents lacked that structure. https://en.wikipedia.org/wiki/Boudica