Hey I may be a bit fat right now but what can I say I am an Ectomesomorph to begin with...
At least I don't have a pony tail and dance on the mat...LOL
On a serious note I think the whole issue is another slander meme foisted upon Aikido by folks who need a reason to dislike it.
That being said being a good shape is essential for practicing any Martial Art if you actually want to enjoy what you're doing.
I know of several Yudansha who have had to get hip replacements ( One very popular Woman had it done and her progress was posted here) and live with some degree of disability...
Believe it or not Aikido is IMHO... An "impact sport"
Now being in good shape may delay the onset on some Aikido related injuries but if you are practicing all aspects of Aikido during class with a certain degree of vigor... You're going get a workout and deal with allot of impact trauma to the body...The effect is CUMULATIVE folks.
Personally I let everyone train to the level they're physically comfortable with and just try to help them overcome their own mental and physical limitations with allot of encouragement. That is the essence of any practice right?
I don't know how many times I have visited another Dojo and the Instructor decided that tonight is going to be "BREAKFALL NIGHT" Ha ha ha ha ha ha!!!
I guess some folks just can't get enough of seeing some big guy fly through the air and hit the mat with the sound of thunder.