This is something I often discuss since I not only run a dojo but I'm involved in fitness as a whole. I don't necessarily agree with your premise but I do think there are a lot of martial artists today who are out of shape.
Good point. It is something I am addressing too, is attitude toward fatter people. Not wanting to create an argument, but rather to express there is validation for the idea that fat people are often not though of as being fit. If you have love handles and lacking six-pack abs many people think you are not fit. This means Aikidoka (what I am getting at) being fatter translates to them not putting enough effort into their training to lose weight. How true or false is that among us?