Damn... R.I.P.
RB is one of the weirdest positions in sports in that all you do is run, most of them are ripped and cut up, but a lot of them are just naturally nimble fat guys who are in shape. Once the training and running ends, all it takes is one cookout too many and your inner fat guy quickly becomes who you are and you look like a totally different person. LBs also fall into this category.
The short stature of most RBs also doesn't help. A stretched out 210 lbs is way different than a bunched up one.
A retired out of shape DB or WR usually still retain somewhat of their look from their playing days, even if they've been living high off the hog for a few years. Most hockey & basketball players age like WRs & DBs as well.