Maybe because they are two different people and not robots off an assembly line. Eddie Lacy was a dominant college back coming off back to back 1100+ yard seasons for GB when he come in overweight last year; not a guy who could never manage to stay in his college team's starting lineup, fell to the fifth round, got hurt halfway through his rookie year and earned himself a 4 game suspension. Really, it's a pretty poor comparison.
Not to mention the fact that Lacy looked like crap for much of last year at 260 and received a ton of heat for it, so that's not exactly a blueprint for success with lesser running backs.