Bulking up isn't always good for a player. Just look look no further than our own Ricky Williams as an example. Heck, I remember Jason Sehorn muscling up to 228 lbs one year. And it affected his play. He just didn't have the speed and agility. That's too big for a CB. Heck, that's big for a safety.
Not only does speed suffer in some cases, but some player's bodies just cannot handle the added weight alone over the long haul. Again, look at Ricky. And some guys' joints just cannot support their ripped up bodies. Some of these guys put on a lot of weight to play. Heck, Brad Culpepper played at 275-285 lbs for his entire 9 year career. One year into retirement, he weighed 190.