The problem I have with that thinking is that for all intents and purposes, USC has a professional training staff, and the athletes are on a full-time weight program. I think that if he could bulk up that much without sacrificing speed, he would have done so already.
He's a player, and he looks to have the long arms you like in an inside backer, but I'm not sold on him holding up in the 3-4 enough to use a top 10 pick on him. If he plays with enough functional strength and leverage, then maybe he can be an exception to the rule that 230-something guys lack the bulk to play inside.
I didn't say anything about taking him in the top ten, actually (in another thread), I said he'd be interesting if they traded down, but the 7th overall pick seemed a little high.
Rivers does have a nicely defined upper body and strength, but I'm not the only person who thinks he could carry some additional weight (a number of scouts have also said he has room to grow on his frame), and he already weighs 236lbs+.
It's not unreasonable to believe a college player will/could gain an additional 10 pounds after entering the NFL, especially when it's well known (among those who've actually scouted him) that said player has room on his frame for additional weight.
Also, we're not talking about USC's defense, but NE's 3-4. He doesn't have to be a blazing sideline-to-sideline LB, and his responsibilities would be somewhat different in NE's defense. I do believe he could add about 10lbs to his frame and still maintain his current speed, but if that weight was to slow him down slightly it wouldn't be a big deal in NE's defense. Remember, we're talking about a guy who (at 236lbs) runs like a Safety.
Maualuga has been a guy I really liked for a couple of year, and Rivers always popped up on my radar, but he was listed at 220lbs (even though he looked bigger to me), so I never really brought him up all that much. After hearing his listed weight at the Senior Bowl I became a little more interested. We'll have to see how he does at the combine and if he gains even more weight prior to the draft, but Rivers just seems like a player to me, and he could be an option at some point in the first round.