I started my fort in 250, and it feels like 80% of my animals are gigantic, with varying amounts of musculature and fat. A few are enormous, and some rare ones don't have a size descriptor (so I guess average?). I don't think the size descriptors are very useful, to be honest. Most gigantic animals are less than 10% larger than the nominal average adult size for their race, and skinny/weak ones can actually be smaller than said average.
Oh, and I've found that it's pretty easy to breed an all-gigantic strain (>90%) of livestock, but getting them all to be "incredible muscles" and/or "untold amounts of lard" or stuff like that is hard. Probably because those seem to be partly lamarkian traits, i.e. they get modified depending on what creatures do (most noticeable with military dwarves slimming down and developing muscle AFAIK), yet might only be stored with one "current" value (as opposed to a separate value for current fat/muscle, and genetic fat/muscle), and thus inheritable.
Previously, I haven't really found anything useful, but now I did! Seems the standard percentages for variation are 90:95:98:100:102:105:110, so not very much; realistically e.g. dogs have been bred for a FAR larger range. E.g. dogs (and apparently most domestic animals) have that variation with length, height, and broadness. Even before looking at the raws, I was starting to come to the conclusion that breeding for gigantic, muscular etc. animals mostly just isn't worth the time/effort. Either your livestock is so small you'll only get 1-2 extra meat etc., or it's so large (yaks and up) that you get so much food from them it doesn't really matter anyway. Also, feeding forts isn't difficult, and bones are a bit like stone in that you usually get more than you can really use... well, usefully.
P.S. Did you know cats can clean themselves and friendlies? Also, all MILKABLE animals (at least domestic ones) have the exact same milking frequency by default, when in reality, cows are the kings queens of milk production, probably followed by yaks etc followed by maybe goats etc. Argh.