Selecting Tua/Lewis as an example only proves the point: Tua walked straight in standing straight up, maximizing his lack of ht and reach, allowing Lewis to use his jab to establish distance fairly easily. Same thing in the Botha fight, for example. Lewis was able to set the distance with that long left hand by keeping it between them.
But if the short guy makes himself shorter, a smaller target, then it is harder to keep that left hand between the two. Bob Foster did it by getting low and jabbing straight to Tiger's chest- keeping him at a difference. This is because jabbing down at a crouching opponent doesn't keep him away nearly so much as it brings him closer and gives him a better chance to punch over the top or to get inside.
Tyson's career proves this. At his peak he came in low and fast, using missed punches to create punching lanes. Against Douglas, when he got countered trying to get past the jab he started standing straight up...and eating jabs. As his career went on he stayed straight up and was just a short guy that hit harde, if he got the chance.
Personally, I have a suspicion that the huge height disparity would make for a dull fight and there would probably be few clean punches landed unless somebody got tired or frustrated and then sloppy. And certainly Marciano would feel the punches of the heavier man.
But just because a guy has a significant size advantage it does not necessarily follow that he will win, especially if the other guy knows how to use what he has as well as Marciano (or Dempsey) did. And, no the world is not littered with super talented 186 pound heavy weights, or super talented fighters in any size shape or form, because there are no longer guys like Charlie Goldman around to teach them to be smart.