It's called muscle. Arreola looks chubby at 245 and Mitchell looks like a tank. Mike Tyson at 5'10 was 237 in his last few fights and he still looked good (fought like ****) but looked good. Banks is a small HW who was a CW.
Wladimir has stick legs, and this guy has American Football legs and probably can squat 3x what Wladimir can?
Both looked about the same size, Mitchell with a little more muscle. I think your right about the additional weight being in his legs.
People very often forget about a simple thing: Bone Structure. Some people have bigger, and heavier bones. It's like they say: You can put on as much muscle as you want - But you can never change the size of your ribcage.
He's just bonier and more muscular. Banks can try to outbox Mitchell all he wants but he's gonna get steamrolled tomorrow, it's that simple. If he makes to the equator, he can consider himself lucky. He has a great full-time job now and probably doesn't give a **** about this fight anyway. I just can't see how he was able to put in needed hours of training being a mitt holder for Wlad for the past month.