Not nearly as much as you'd think, not unless the skill-sets are relatively similar. Size does, that's why we have weight classes. However, the larger the fighters in general, the less that disparity becomes. A 200 pound boxer is more than capable of KO'ing a 250 pound one, whereas a 100 pound boxer isn't very likely to be KO'ing a 150 pound one. Basketball was in its infancy on a global scale in the early 40's, by the 70's and 80's it had hit its stride. That's 40 years tops. Boxing was in its infancy on a global scale in the 1890's, and by the 30's and 40's it had hit its own stride. That's 50 years. It takes time for a sport to evolve, especially when the rules and regulations of a sport change so drastically over time, as boxing did post 1920 with the inception of the Walker Law (which gave us standard weight classes, the regulatory commission, banned the use of roughing tactics, limited fights to 15 rounds, etc). Lewis had a 4 inch height advantage over Dempsey, and see my first part in response to the weight differential. Still, I don't think Dempsey was quite evolved enough by modern era standards to take out Lewis, given the disparity in physical attributes. I doubt it'd be over in one round, but I agree with you that Lewis would take the fight. Boxing hadn't peaked in Dempsey's day, although his berserker style would probably be one of the best of his era for off-setting the technical skills of the more advanced modern day Heavyweights to come after him. He was a swarmer, and a massive puncher. We have those in every era, and they're the primary style for throwing off the fight plans of more reserved, cautious boxers like Lewis.
You have trouble understanding what you read. I can show clips from today's top heavyweights that would make them seem awful. Did you not watch Klitschko-Haye? The point is simple, and I hope you'll get it this time. Lewis beats Dempsey because he's better than Dempsey. He loses to Joe Louis because he's not as good as Louis. It has little to do with when they were alive. I can show you clips from those days in which the old guys look like masters.
People on here sometimes claim the old guys would have an advantage if old rules were used, but imagine Lewis being allowed to punch a guy as he is picking himself up off the canvas whilst wearing MMA sized gloves? He'd kill other fighters. Or if tall fighters like Lewis and the Klitschkos could just hold back other fighters with an outstretched arm like Jack Johnson did? I don't buy the idea that skills haven't developed in boxing. To claim that suggests guys like Ali didn't advance the sport in any way, or that trainers haven't watched any of the fights that have happened in the last 50-70 years to pick up new strategies and techniques
Klitschko-Haye showed big guys who both have a combination of handspeed, footspeed, upper body movement and power the likes of which was never seen before the Ali generation
That's the thing, the trainers from the sport's golden age (the 40's through the 80's) were the ones that had lived those times, seen the peak of boxing. They taught the fighters these techniques. They didn't need to rely on videotape because it was first hand knowledge for them, ingrained into their senses. Guys like Eddie Futch (who was one of the most accomplished trainers in the business up until his recent death, laying the blueprint for some of the biggest upsets in the sport) got their tutelage back in the days of the Black Murderer's Row, in the 30's and 40's. He is one of the premier technical minds in the history of the game, and that's where he learned his craft. Up until his death he would tell you he never saw better fighters than Charley Burley, or that he'd rather watch Holman Williams shadow-box than any of today's fighters actually fight. What does that say to you? In reality, as the sport's old breed of trainers die out, the techniques of the past die with them. If you disagree, name one new facet to the technical side of the game we've learned in the past 30-40 years? I could name you quite a few from yesteryear that have gone largely under the radar in recent years, by comparison (certain fighters aside).
Wlad is the exception to the rule though. In the history of super heavyweights, only three have been genuinely good fighters: Lewis, Bowe and Wlad. The rest simply weren't and I can't see that changing much either. When fighters get beyond 6'4" and 240lbs, they suffer in certain areas, which are as important as any which their size benefits them. For example, a fighter over that measurement is unlikely to be agile and whilst he may have more reach, which is more important to a fighter, agility or reach? I think there is limit for heavyweights to be effective. Even though the weight division has no limit, there is a limit on how effective big guys are.
Just to further add on to my point, I think the biggest thing which hurts super heavyweights is stamina. There will never, ever be a super heavyweight who can go at the same pace that Joe Frazier or Rocky Marciano could. They couldn't even do it for a round or two, never mind 15 hard rounds like we saw from Frazier against Ali. When Wlad lost to Brewster, he was completely shattered after 6 rounds of punching and whilst he threw a lot of punches, he wasn't going crazy but it was a hard enough pace to give Brewster the win. A chin was all he needed to beat Wlad, because he knew Wlad would punch himself out if stayed right in front of him. A fighter like Frazier could go at a much harder pace than that and fight it for 15 rounds, and still be fit at the end of the fight. He wouldn't be gassed after less than 20 minutes and stumbling all over the place. Size, like most things in boxing, has to be a compromise. Having more and more isn't the answer to being a better fighter because the more size a fighter has, the weaker he will be in other areas and stamina is a key one of those, I think. An agile, fast, tough, skilled super heavyweight with bags of stamina probably is the perfect fighter. However, I don't think one will ever exist because it goes beyond the physical limitations of humans to have all those attributes.
Anyone who believes that Dempsey takes Lewis should go out of their way to kill themselves at their earliest convenience.
I think Lewis was pretty close to being a tough skilled super heavyweight with good stamina. Dont remember him getting tired too much.