As the original poster on this thread, I feel that I owe most posters an acknowledgement and thanks. The discussion seems to have got a bit off track in talking about which aircraft Qantas assign to which flights, but never-the-less most posts have tried to contribute to the topic under discussion. Thanks to markis10, Boris spatsky, drron and simonsyd, all of whom have contributed specialist knowledge. I thank Rooflyer, too, for his support for my concerns. To Whatmeworry, you suggest that I should have used the descriptor "morbidly obese". You're dead right. I believe that "morbidly obese" actually has some technical definition, so I didn't use it, but the pilot in question certainly fitted the common usage of that term. In fact, he was worse than the cartoon of Homer Simpson which you helpfully supplied.
Nalagalle, you intimated that there were plenty of Tommy Hafeys among our octogenarians (or at least that's the way I read your comment), but I'd put it to you that Tommy is a stand out and virtually unique in Australia among his age group. He is the exception proving the rule!
In closing, I'd just like to stress that I was talking about extreme cases - those very much over weight and the obviously elderly. Also, I was focussing particularly on the situation in USA, where these extreme cases seem to be more in evidence. Some of you talked about Qantas crew. I have seen not a single case on Qantas which I would describe as doubtful. I have not the least reason to be dissatisfied with their crew selection and training.
Happy and safe flying, all.
Cocitus23