It's worth noting that, while Greek and Roman soldiers wouldn't look like the characters in those movies, six-packs and large muscles were definitely in existence back then, and were considered as attractive (and as rare) as they are today. Just look at any number of statues of heroes and gods from the period, and you'll see the familiar musculature of a modern actor or model:
These masculine forms were still a little bit bulkier than today's styles would prefer (look up some Abercrombe ads and you'll see mostly longer, thinner torsos), but such is the nature of fashion.
The main thing to consider here is practicality: the Captain America physique takes a LOT of training and effort to maintain, without actually giving that many benefits above similar levels of strength but with slightly higher body fat percentages. It looks great on camera, but few professional soldiers are going to want to put in the extra effort to get there for no reason.
Go to any real army base, firestation, or construction site today, and you'll see basically the same thing: a few guys who go all-out to look perfect, and the rest who are very strong but with less chiseled definition due to their higher body fat ratios. (It's worth noting that word "chiseled" to indicate highly defined muscles, and its connection to chiseling a statue.) I've seen soldiers who looked straight-up fat, but who could seriously throw down because they were still incredibly strong under the top layer.
The actors in those movies are essentially male models, and their workout regimen is aimed at making them look fantastic. The ancients had the same impulse for their statues and artwork. But the next time you see an image of real soldiers with their shirts off, whether they're from World War 2 or Vietnam or Afghanistan or Iraq, you'll be getting a better glimpse at what soldiers throughout the ages have looked like: strong, powerful, well-maintained, but also (hopefully) a little more well-fed.