Yeah, Tyson was quick; for about the first 4 rounds that is. Tyson's speed came at a price you know; the greater your body mass, the more energy you spend moving at high speed. Consider how much fuel Tanks and Jet airplanes burn compared to, say, a mini cooper. Also consider how much more fuel Tanks and Jet airplanes burn when traveling at top speed. The greater the body mass, the more energy you spend, and boxers need stamina. Its not that Tyson did not have any stamina, it was more the fact that he tended to waste his energy. He only got away with it, because he was born in an era of boxing where he could get away with it, had he been born in earlier days, specifically the golden age, no way in hell would he have gotten away with it then.
Its not that the champs of old were smaller; it was more the fact that dudes over 220 lbs rarely made it to the top 10's. Let me tell you a Marciano story, involving a boxer from Idaho. Idahoans, like many people in the pacific northwest, the men often average around 6'1 or 2, weighing at around 230 lbs sometimes (its all natural bulk too), while the women often average at around 5'10 or so. There are some big people around here.
The Idahoan in question Marciano fought, stood at 6'5, weighing in at 270 lbs of solid muscle. He, was a big man, and he was no slouch either, he had 40 wins and 2 losses under his belt when he fought Marciano. However, when he went up against Rocky, as said fighter was on his way up, Marciano dropped him in the 3rd round with a ferocious left hook that broke the poor man's jaw. The reason Marciano was able to do it, is because the dude was so big, by the third stinking round he was already starting to get winded and therefore unable to keep his guard up.
Heavyweights did not shrink man; its just that in the olden days they simply didn't make it, as all the 190 to 200 lb guys often out boxed, and out lasted them. Observe Liston, and also observe Foreman; being big men, you will notice that both those fighters were wise to conserve their energy, and often fought defensively and conservatively whereas Tyson well, we all know his story, he tended to explode all over his man's face in the first round and as we also know, that often ended his night. Quick question though, after those bouts of unreal fury, did he even have enough left in his tank to even f*ck someone? I think not; try throwing punches for 3 minutes straight with Tyson's fury and see how much YOU have left in your tank.
How was Tyson able to move so quick? By spending 15 round's worth of energy in 1 that's how. Ultimately though everything's a tradeoff; big dudes have power but no stamina. Little dudes can last into overtime but they hit like school girls and their fights are not fun to watch for audiences thirsty for blood, and middle weights, true to their size, tend to be in the middle, neither marathoners, but not human wrecking balls either.
The middle weight division is enjoying a "golden age" in its own right as of late, largely because the heavies are not as skilled as they once were, and boxing fans are getting sick of watching those robotic Russians. Boxing fans want a champ who is natural, with "soul" if you will, not someone who fights like a machine.