It could just be that Dorn worked out a lot in the TNG run. But his build in Heart of Glory relative to Birthright Pt. II is noticeable. Even still, in both TNG and DS9 he was never
The only fat actor/actress ever in Trek was James Doohan (in the films at least).
But Dorn has aged better than Frakes. I saw "Captain's Chair" the other day, and boy he's grown.