The degree of fluffiness will also depend on the type and source of the chicken within a breed. Hatchery stock tend to be less luscious and fluffy than breeder stock, hatchery chickens tend to look more "ordinary" than what the true breed calls for. With Orpingtons there's also a big difference between English style and American style. Hatcheries usually sell American style, which are slimmer and less fluffy and not much different than just generic chickens in terms of shape and fluff. English Orpingtons that come from a breeder, on the other hand, can be as round as a basketball and extremely fluffy. I've had both hatchery stock American Orpingtons and breeder stock English Orpingtons and they are like two completely different breeds! The hatchery Orps do fine in the summer, but my English basketballs suffer quite a lot - they pant and spread their wings out and get sluggish and just miserable (we have frequent 90-degree stretches and really high humidity in the summer). I haven't had breeder stock Wyandottes so no personal experience, but with the hatchery quality Wyandottes I have had I've noticed a similar difference - they don't get as round and fluffy as they are supposed to per breed standard. And mine have been fine in the hot summers. So, if you want to be on the safe side, get yourself some generic hatchery Wyandottes that won't get thick and fluffy, and they'll probably be fine.