The only fighters I can think of who have won world titles in 3 divisions above middleweight are Bob Fitzsimmons, Roy Jones Jr and James Toney. There are however plenty who have achieved this and more in the lower divisions. Clearly there appears to be a reluctance among some of the bigger guys around today to step up and face opponents in higher divisions. Does it become proportionately more difficult to move up in weight the heavier you get?
Lower weight classes are only a few pounds apart. For example 118, 122, 126 (8 pounds) 160, 168, 175 (15 pounds)
And here we have it. The jump to cruiser is then another 25 pounds before heavyweight wherein you'll face the likes of Wlad at a lean 240.
Also, in the lower classes you get the "growing in the weight" thing. When I was just a little Robney I weighed a mere 176lbs. Now, I would have to completely starve and drain myself to maybe make it down to 190, but there isn't a weightclass there, so Cruiser would be the bare minimum for me. In the midget weightclasses young guys can start out at 105 and bulk all the way upto 126, wich is 7 weightclasses.