Pound-for-pound ranking is a hypothetical ranking system which aims to evaluate fighters’ rankings if the variable of size was to be eliminated. The UFC, ESPN, Sherdog and MMA Junkie pound-for-pound rankings all have Jon Jones as number 1 and Cain Velasquez as number 5. This is astonishing given then criterion mentioned above. Jon Jones is pretty big for his weight division to the point where there is constantly talk of him moving up a weight division whereas Cain Velasquez, on the other hand, is pretty small for his weight division. This is notwithstanding the fact that many of Jon Jone’s opponents were so much smaller than him that they are now, or are usually known to be, middleweights (namely; Lyoto Machida, Vitor Belfort, Chael Sonnen and Andre Gusmao), others beat by current middleweights (namely; Jake O’Brien by Gegard Mousasi, Matt Hamill by Michael Bisping, Ryan Bader and Rashad Evans by Lyoto Machida and Stephan Bonnar by Anderson Silva) and others talking about moving down to middleweight (namely Mauricio Rua). Even when he did fight ‘big’ fighters, he was either still bigger (namely Daniel Cormier - in fact Mike Dolce said Cormier could make middleweight without his performance suffering) or they were just low/mid-level fighters (namely Vladimir Matyushenko, Brandon Vera and Quinton Jackson) or where he won via unanimous decision, sometimes quite unimpressively (namely; Alexander Gustafsson and Gloveria Teixeria). By the way, these are all of Jon Jone’s UFC fights!- I've posted Jone's record from this perspective in table format.
On the other hand, Cain Velasquez looks like he can go down to middleweight if he cuts down but he is at the heavyweight division finishing the majority of his fights or winning in very impressive fashion. This does not mean Velasquez would beat Jones if they fought, but it does mean until that fight happens the former should be above the latter in the P4P rankings. And it is true that many of Velasquez’s opponents are not high level fighters but they are either same size or bigger whereas all Jone’s opponents are either smaller or low/mid-level and if they are not, he has struggled or won in regular fashion at best. Once again, P4P is a hypothetical ranking and we may never know who really is P4P best but fights like Jones vs Velasquez sure do make things easier.