10. Georges St-Pierre (4 Years To Rebuild)
When discussing the greatest pound for pound fighters to ever step inside the unforgiving 8-sided structure, Georges St-Pierre will top many a list.
The man was a two time Welterweight Champion and defended the belt a mind-boggling nine times - the most consecutive defences in the division. He also claimed a Middleweight title and has had more comebacks than Robbie Williams.
In his most recent return in 2017, GSP moved up a weight class, from Welterweight to Middleweight, and packed a decent amount of muscle onto his usually slender frame.
If you look at GSP in his last weigh-in at Welterweight, way back in 2013, you see a man who looks as though he has been crafted by an artist. Fast forward to 2017 and the former champ looked as though he had sacrificed that definition in order to pack the sort of power needed to be dominant at Middleweight.
It worked. Gaining 15 pounds, GSP clinched the title from Michael Bisping by submission in his first bout at the weight.
It doesn't rank as one of the most drastic physical transformations in UFC history, but in terms of effectiveness, you can't really argue with one major weight gain resulting in one major win.