Hard to say without a better idea of Sullivan's style in gloved bouts. I have read a few contemporary acounts and not realy got much closer to understanding it.
Pound for pound Fitz might have been better, but the bottom line is that Sullivan was way too big, strong, and fast for Bob. Sullivan was a fast brutal puncher with a good chin and ferocious style, and at over 200 pounds would simply be too damn big and strong for a guy who was typically no more than 170 pounds even as a heavyweight. Fitz was able to hang in there with Corbett because Jim was not a killer puncher and only weighed in the 180-pound range himself. True Fitz KO'd a number of 200-plus-pound guys, but not top of the line boxers. When he got in there with a big guy with some talent (Jeffries) he got KO'd both times. At some point, size does tell. However, there were those who said that a prime Bob would KO an over the hill Sullivan, at least the one that fought Corbett.
Fitzsimmons was a shade taller. The reach of the two men were about the same. Fitzsimmons hit harder, and according to the papers had cobra like hand speed and could strike like lighting. When I review the records of the two men, it seems that Fitzsimons was the better knock out artist by far. Fitzsimmons had paralyzing power, and was genuine life taker. He killed two men in the ring in prize fights, and another in a sparring session. Fighters were genuinely terrified of Fitzsimmons hitting power. There is no doubt that Fitzsimmons met and defeated better fighters than Sullivan did. Size was never a problem for Ruby Robert. Indeed it was Fitzsimmons who coined the phrase, the bigger than are the harder they fall. Fighters who hit hard but lacked defense such as Maher or Sharkey were quickly pole axed by Fitzsimmons. How good Sullivan really was and how much he could take remains a mystery. I cant help but notice he had trouble vs Charley Mitchell and wasnt much of a knockout artists vs the best he fought. I think Fitzsimmons would defeat Sullivan.
Mendoza, although I disagree, you make a good argument. If you carefully review Sullivan's record of very fast and brutal knockouts (a LOT of them), I think you will revise your opinion about his punching power. Read John L. Sullivan: The Career of the First Gloved Heavyweight Champion, and you will see how much in awe of him the public was. When there were no legal regulations and constraints (or LPR rules which allowed for long-term survival), Sully bashed the living hell out of his opponents, including Mitchell. As for the phrase, 'the bigger they are the harder they fall,' it is a common historical misbelief that Fitz coined that phrase. John L. Sullivan actually said it before Fitzsimmons, and there is primary source evidence of it.
if the author of the books on sullivan and to come, fitzsimmons believes sullivan would win, it's hard to debunk it. I also think that at some point there's too much of a size difference, and that would be fitz's problems. he might do well for a short time, but in a slugfest sullivan probably overwhelms him. as with any of these phantasy matchups, it's also possible fitz catches sullivan and lays him out, there's always a chance the other guy could win, somehow.
Sullivan probably hit harder as a HW, bigger, stronger, faster, look at my avatar how he looks, he would overwhelm Fitz!
