Gawd, that scene with a bloated, red-faced Riker toasting their dead (not really) comrade Lt. Data was cinematically pathetic (as we still had Data Gump). And Picard looked so frail and small onscreen.
And...Jonathan Frakes' age, health and appearance at the time
Nemesis was filmed and Stewart's physical stature have to do with the greater film as a whole...
how, again?
Actors age. By the time the tenth movie (and the fourth one with the TNG cast) saw cameras roll, it had been fifteen years since the pilot for
Next Gen and they - well, you probably guessed where I'm going with this -got older. Yes, Frakes gained some weight and got puffier. That's going to happen to quite a few of us. He was no longer as trim and fit as he had been during his thirties and the premiere season of the series, and Patrick Stewart aged fifteen years as well. He was already in his late forties when the show began and was approaching senior citizen status during
Nemesis.
To echo what somebody earlier in the thread said, we might as well go ahead and fire Simon Pegg, Chris Pine and Karl Urban now, because they're all just going to get older, wrinklier and slower if the new films are successful enough to keep going for a few more movies or even longer.
There are a hundred very valid things to nitpick and even slam about
Nemesis, but Will Riker's face and Patrick Stewart not sporting a strong Zapp Brannigan physique (when he's never been a large, burly actor or character and Picard is the age of 74 by that year in the timeline) shouldn't be two of them.