How often do smaller heavyweights defeat bigger heavyweights?

It's 50-50

Often enough that the heavyweight champion isn't now 7 foot tall. There's a cycle in boxing. Good big 'un beats a good little 'un. Until the the big 'uns get so big that they get made to look clumsy by a smaller, faster fighter. Size is a big advantage until it becomes a disadvantage.

often enough for people to mostly stop saying that guy has no chance hes too small

good question When does additional size backfire on average