He didn't gain the weight to be a power back, he gained weight to hold up better over the course of the season.
I think his injuries are more of a factor right now in limiting his quickness and burst.
Gibbs has used many different types of backs over his coaching career, yet people seem to think a power back is the only back for his system. Smaller backs like Ricky Ervins and Joe Washington had success in his system, even Earnest Byner wasn't what you would call a true power back.
Failed experiment? After one 1300+ yard season and this season where he's on a similar pace we're calling that a failure?