I'm terrible at editing my posts and it would be a wall anyway if I tried so Im just going to number my responses to your points:
1. A valid point. I'll be honest, the Au Ra females don't look to be particularly well muscled. However there is a really noticeable difference between endurance and burst strength in bulk. The Au Ra males are actually probably pretty accurate I suspect in muscle definition.
2. From personal experience I can tell you outright that in muddy conditions being large makes movement far more difficult and risky than being smaller. Its simple physics really. I think my point though was that a smaller foe can use conditions and terrain to their own benefit. Smaller doesn't necessarily mean worse in combat unless the difference is huge. Smaller can also sometimes mean being a smaller target.
3. Are you talking about in modern times or historically. The average high of humans has been increasing steadily since the Agricultural revolution. Simple reality is more food means bigger people and not just fatter. The extra protein during youth substantially increases growth and size. We are also talking about a resource poor landscape. I cant speak for the Native Americans but if we look at Mongolians which lived in a terrain closer to what the Xeala are supposed to live in then we see a short group of people. In fact even their horses were short. The reality of Hunter Gatherer societies was they simply had far less food supply due to the effort it took to acquire it and burned far more energy in the process of acquiring it.
4. Im going to be honest here. It actually wouldn't bother me if the Au Ra females were bigger. We know that SE made them that way because they felt they would appeal to a larger audience. I just lept into the debate because I like arguing over technicalities in logic. :P
5. You clearly haven't known many farmers or labourers if you think the Raen wouldn't be buff. And this is a time prior to machines to help with the labour. I actually think it would be more fun to have the thin, waifish ones the bloodthirsty types but that's just me. :P