As long as good form is maintained and the weight is a decent amount over the individual's body weight than more power to them. Besides just the simple weight factor why is the last statment above true? I know your right, it only makes sense but if you look at the numbers and such the 300lber should be able to advance as quickly as the 150lber. If you take away the fact the lighter person will be doing less weight look at it from the other side. The 300lber has more body mass, more muscle mass, more stability, and more acomadated to move the heavier weight. With all that said I guess the main problem is that damn thing called gravity. 600lbs is pretty damn heavy any way you look at it.