Total mixed bag, not really weight, physics related at all, because if it was you wouldn't have guys being pancaked by Aaron Rodgers etc. It's really random, sometimes it looks good, sometimes you shake your head.
In short, if you're asking does the engine take each collision and calculate, angle, weight, power, physics properly, the answer is a resounding NO. Does it ruin the game, no, not at all. It's just not a very fleshed out or accurate weight/collision model.