Maybe heavier guys do get more knockouts, but I am willing to bet that lighter guys get a lot more submissions. So your switch of subject to say knockouts is a suitable substitute for finishes doesn't work.
that maybe true for the lower ranked guys, but title figths don't end in submission nearly as much. so if you lack ko power and your only way to finish a fight is by submisson. you're not going to be getting the same amount of finishes in title matches as people with knockout power.
lets look at the current ufc champs finishes in title fights and the guy who preceded them
cain's title fights - 0 finishes
JDS's title figths - 2 ko's/tko's
JBJ title fights - 1 ko/tko, 3 submission
shogun title fights - 1ko/tko
anderson's title fights - 7 ko's/tko's, 3 submission
rich's title fights - 2 ko's/tko's
gsp's title fights - 3 ko's/tko's, 1 submission
serra's title figths - 1 ko
benson henderson title fights - 0 finishes
frankie edgar title fights - 1 ko/tko
jose aldo - 1 ko
dominic cruz - 0 finishes
demetrious johnson - 0 finishes
so 19 knockouts vs 7 submissions.
in title fights at the top level submissions are less prevalent. and since gsp doesn't have alot of natural punching power and since title fights is where gsp has been fighting recently his ratio has gone to shit. but if he had natural power like cain did it would be a compeletely different story
if you want to compare gsp while he was beating up guys who weren't championship calibre his finishing ratio goes up alot, just like most of cains finishes come outside of title fights against guys like big ben and dennis stoynic