Big vs. Small in a boxing match?

I keep hearing that a big man beats a small man all else equal. That is true up to heavyweight. Once a man gets to be 190 or so, if he can swat, he can knock out a man of any size. That's why the boxing people stopped graduating the cles at 175 (and now 190). If it kept making a difference, they would have about 5 more weight cles. Also, Wladimir Klitschko would be favored to beat the hell out of Ali, Foreman, Liston, Marciano, Louis, etc. when it comes to heavyweights, size doesn't matter! Whaddya think?