Miller has been playing CB and safety, so he wasn't trying to bulk up at all. 205 is a pretty big weight for a DB. Now that he has been moved to LB, he can start -now- to put weight on...assuming the move is permanent.
We have depth at safety, and little at LB, so the move may have been out of necessity, but like a lot of people, I think its his more natural position. While his speed may not have been overwhelming for a DB, he will be a really fast LB, even if he puts on some weight.
He is 6-2 to 6-3, so he has the frame to put on weight without losing any speed. Zeise is an inch or two shorter, and he put on 20 lbs or so when he moved to LB and he appears to have the same speed.
Good move.