I disagree with every single generalization that was made. You can be fat, but it is not advisable. I can find fat practioners in every art. Other than that, I won't do a point by point counter!
Agree with Jo's statement above!
Interestingly enough, I've seen students leave our Aikido school to try out other martial arts. For example a fella left for a few months and was training in some standing jujitsu and some southern style form of karate(forget the exact name of it). He came back to visit and train with us, because we keep friendly with everyone who walks in our doors mostly, and he was sweating his butt off, needing to take breaks in the middle of training. He said, "I forgot how much Aikido takes out of you! Practicing at my other dojo I'd barely ever wash my gi because I'd rarely sweat, but at this dojo I'm soaked!"
Fat or nonathletic... it really depends on the dojo. You can practice Aikido I think in a way that is low key... or you can throw down with the young bucks and get yourself some a dang good work out. Ukemi forms to fit the nage in my opinion. Most of my school's ukemi style actually requires a fair deal of flexibility, agility and cardio training to pull off... that's what the nage requires from their uke though in my school. It is different everywhere. Generalizations suck!