I feel this thread has lost its way slightly. I think it is a fair point raised by GW, perhaps not phrased in the best way though. I would also wholeheartedly agree that thin does not necessarily equal competent, and fat equal incompetent. The real argument as I see it is one of self discipline. There is a reason for required fitness levels across the military, and this includes obesity. I am afraid you do not need to be a trained clinician to spot obesity, and yes I will agree there are some very large people, who remain very fit. This however is more of an exception than the rule, and very few fall into this category. I am afraid to say that in general - if they look obese, they probably are, and I can say this as it is a field in which I am trained.
Good physical fitness is directly related to overall good health, physical and just as importantly - mental. Operational environments create a huge amount of stress on the body, due to a number of factors. Long working hours, night flying and irregular sleep patterns, as well as environmental conditions, all add put a great strain on the mind and body, affecting many systems, including the immune sysytem, nervous system, and digestive systems. The evidence is overwhelming and indisputible that good physical fitness is the best way to cope with this. This does not mean being a marathon runner, but simple, regular exercise, weight under control and a healthy diet - no excuses.
I will agree that experience is very important, and these experienced individuals will understand the need to be best prepared for the job in all ways - mentally as well as physically. Being overweight means you are far more prone to illness, stress related issues, injury and feeling the need to defend your size! Anyone can train and get fit. If they cant, then they should be downgraded and removed placed in rehab, as they present a liability and burden to their crew and operation.
I dont intend to preach and appreciate the experience argument, as well as the fact that this may be sometimes unachievable, but their must be targets and every attempt made to reach them.