One way to look at the OP is to start with an extreme case, and then whittle away at it and see what we end up with.
Here's what I mean: let's start with a hypothetical boxing match between a featherweight champ and one of the Klitschko brothers. Can the featherweight beat Klitschko? I can assume we'd all say "no", right? For now, let's ignore that the featherweight might be able to avoid Klitschko all night and avoid getting hit -- featherweight would still lose on a decision.
OK ... now let's take the same two guys in a UFC-style match. I hope we can all agree that Klitschko wins with little effort.
Now, what's the point? Let's take away of Klitschko's ability and experience, and move him closer to the size of an average guy. How does a guy with 95% of Klitschko's experience/athleticism/size/etc fare against the featherweight?
Answer that in your head and keep working down. Where's the breaking point where the featherweight finally has the advantage on the larger opponent?
Something else to consider -- some say boxing is a skill. And it is. Some say you've got to be "in boxing shape". And you do ... usually. But can some other kind of shape make up for it? Let's take Dan Severn from the early UFC days. IIRC, the guy had almost no striking background at the time, but was a highly accomplished wrestler. Severn was around 270 lbs, but wasn't super tall (6 ft even? Maybe 6'1"?). Could the featherweight champ take out Severn in a boxing match, with boxing rules? I'm not so sure ... it would probably often end in a decision for the featherweight, who would land blows, but none with enough force to down Severn. So how much does Severn's lack of boxing skill cost him if he's still standing and ready to rumble after 12 rounds? Would it be the wrestling background that helps Severn carry on and stand in, thereby suggesting that some kind of combat training can substitute for boxing skill?