I have to agree with isatarak. I think the main benefit of larger cuts would be an increased chance of hitting the vitals. I really don't think that a few 16ths of an inch would make much difference in the distance a deer will run.

If shot in the same spot, yeah maybe, but i think a fixed blade broadhead has the advantage of not losing much k.e. upon impact as opposed to the mechanical. With a lesser poundage bow, a mech. might not do as good as a fixed at a 25 yard shot right behind the shoulder. (but this is just from what i have seen )