Not trying to take anything away from Cain, but he doesnt really have any impressive wins, 1 good win against a declining Nog isnt that good. He beat Kongo and Ben Rothwell, both suck on the ground and Rothwell was REALLY slow in that fight.
Cain could easily make LHW if he actually tried and went on a diet/conditioning program like most fighters. I am not saying he doesnt train hard, or that his work ethic is bad, just that he hardly has any muscle mass compared to other fighters, and he probably eats anything he wants like most HWs.
He has some belly fat also, which isnt exactly a bad thing, but these type of things can be gotten rid of to make the cut to LHW. I really dont think Cain has the size to compete with future HW opponents, most noticeably, Lesnar.
The way I feel about Cain is that he will most likely never hold the HW title as long as Lesnar is the champ, while he could probably be a LHW champion earlier in his career if he would make the move.