Exercise is good for a variety of reasons. It will burn some additional calories. But most scientists that specialize in metabolic research believe that the constrained total energy expenditure model and metabolic adaption are at least broadly true. You can't exercise out of a bad diet - and it's difficult even if you believe in a fully additive energy expenditure model.
Diet is the overwhelming deciding factor when it comes to weight gain.
Diet is the overwhelming deciding factor when it comes to weight gain.