It's certainly true there have been no hugely overweight presidents since at least the dawn of the television age and not many even before that. But why? Here are some possible reasons.
Fat bigotry. Being overweight doesn't play well in the media and to win elections you need to play well in the media. That isn't necessarily the media's fault, but we seem to favor people who look good. And, for better or worse, being overweight is considered points off in looking good.Fat bigotry is probably the biggest part of the answer and may even motivate the workout routines to stay in shape and not become less appealing in the media. Stress probably only plays a minor part since the presidents likely would not have gotten to the stressful position had they been overweight in the first place.
Workouts. Overachievers apply their overachiever-ness to their health as well. It's common, and almost required, that modern presidents workout. Bush cycles, Clinton ran, Obama bowls, er, plays basketball.
Stress. All that campaigning and governing is stressful. Stress often helps keep the weight off even if for the wrong reasons.
So anyone have a better answer?