Marciano, like Dempsey and indeed Frazier would be considered way to small to be modern heavyweights. Perhaps they would still have beaten plenty but presimably it would have been harder for them.
As has been said Marciano did beat Joe Louis, but Louis was way past his best by then. Had they met when Louis was at his peak my dad reckoned Louis would have won easily.
Another change in boxing, apart from fighters getting bigger, is that they go on for longer. A boxer can comfortably perform at 35, and in some cases older, now days, but in Louis' day, and even Ali's, they would have usually been regarded as over the hill by then.