The problem with the character of Riker was, frankly, Patrick Stewart.
TNG was originally conceived with the concept of the captain not being sent down to the planet in the thick of things every single week, as was done on TOS. David Gerrold, among others, originally pointed out the folly of putting your captain in mortal danger on a weekly basis, made all the worse by him normally having his first officer with him at the same time.
Picard was to be the thinker, the diplomat, and the decision-maker. His choices were to drive the story. But he was not "action guy". He would not, except in unusual circumstances, be down on the planet getting in the phaser fights with the baddie of the week. That would be Riker.
That format held pretty true through the first couple of seasons, but once TNG was a bona fide hit, and Patrick Stewart had a lot more clout, he started to express his desire that the captain needed more "shooting and sex". Stewart is a great actor, but it troubles me that he couldn't see the potential for him as an actor to play Picard as originally written, but needed to focus on getting him into the Kirkian action and love scenes.
Gradually, they kept moving more in the direction of putting Picard in the center of things, and Riker's role became less clear. By the time of the movies, particularly First Contact where Stewart threw a fit over the idea that he'd be helping Cochrane while Riker fought the Borg, the transition was complete and we ended up with "action hero Picard".
It's a shame, because I thought the original characterizations were much more interesting.
EDIT to add: It also disappoints me that we're talking in this thread about the actors "getting fat". It shows me just how skewed our perceptions have become in society. There was not a single person, even in Nemesis, among the TNG cast who "got fat". Sure, they had a few more pounds than they did in their series days -- you know, as people normally do when they get older -- but to call them "fat" only reinforces our ridiculous anorexic Hollywood standard that our society loves to embrace.