[wynand]

There is a relationship between exercise and weight but not nearly as big as people like to believe. You're right that diet has a much bigger impact.

I had to dig out this paper the other day because someone that I was talking to did not want to believe that weight is mainly tied to one's diet:

http://www.nature.com/?file=%2Fijo%2Fjournal%2Fv21%2Fn10%2Fa...

I'm not sure if this is behind a paywall (my uni has access to everything on Nature's site), so here is the salient bit:

RESULTS: Primarily, subjects aged 40 y have been studied (39.5±0.4 y, mean±s.e.m.) who are only moderately obese (92.7±0.9 kg, 33.2±0.5 body mass index (BMI), 33.4±0.7% body fat); for short durations (15.6±0.6 weeks). Exercise studies were of a shorter duration, used younger subjects who weighed less, had lower BMI and percentage body fat values, than diet or diet plus exercise studies. Despite these differences, weight lost through diet, exercise and diet plus exercise was 10.7±0.5, 2.9±0.4* and 11.0±0.6 kg, respectively.