Hello,
So I know people used to be smaller and I know it depends on locale (I think south americans had soy beans), therefore lets say in Europe or the middle east... People had strong muscles, they had to in order to maintain life, but protein sources were quite limited for the low classes (and please correct if I'm utterly wrong here). All the protein sources I can think of are meat (which was rare), milk (which is limited in protein, plus I suppose there was a cap on it as well, certainly for the levels one need to drink in order to compare to meat), eggs, and various plants which are only available in our globalized world. Did they have more food than I think? Or is that little food is enough for bodybuilding? (in contrast to many trainers food recommendations).
Thank you.
They weren't smaller in particular areas.
The medieval english were 'average'. Henry the 8th was a pretty big bloke. (6' + 2" ish supposed to be in and around the 20 stone mark. (around 115 kg) you can tell from his armour).
Milk btw is pure protein and fat. (which is why it's milk, no animal rasies offspring on a fat or protein limited diet).
If you want to check out a medieval cook book take a peek at the Forme of Curry (form of cooking)
(they basically ate everything that lived breathed or crawled)
Average diet and famine on the other hand are different things.
It will be hard to make generalizations about this. Note, though, that while ancient and medieval people might have been physically very fit, they weren't necessarily muscular by modern standards. A strong muscle is not necessarily a big muscle (compare the physiques of Olympic lifters and world-class bodybuilders: the Olympic lifters are smaller, but pound for pound they are stronger). The body's protein "needs" are actually quite low, anyway. The average medieval peasant wasn't as strong as a modern powerlifter, but s/he didn't need to be.
Different peoples subsisted on different diets, but it's hard to know the details. The prehistoric European diet was quite high in meat: it stands to reason that tribal or pre-agricultural peoples would've kept this diet. We're told (by Caesar) that the Britons subsisted on meat and milk, which helped make them large: in comparison the Roman diet was higher in grain. I believe Tacitus says something similar about the German diet, although he had an obvious political agenda in describing the Germans.
It's a bit mistaken to assert that protein would've been rare or otherwise hard to come by. Hunting for one's own food has been a common practice at various times, and lower population density would've made this slightly more sustainable on a large scale.