That is VERY quick. The average muscle gain per year is 10lbs. And that is solid muscle, not counting fat. Also take into account that if you gain 10lbs your first year, it doesn't mean in 10 years you'll gain 100lbs. It doesn't work that way. There comes a point where your body just will not produce any more muscle.
Also take into consideration injuries. In lifting weights sometimes you suffer injuries even doing everything perfect. And those injuries set you back (pretty much what brought my weight lifting days to a halt). But he's also a fighter so he will suffer even more injuries which will set him back even further. So those 3 years, isn't like a solid 3 years of straight weight training. He has to overcome injuries and ALSO train for his fights.
So yeah, 3 years is pretty damn quick to put on 60 lbs of solid muscle. Even the announcer in K-1 said he looked bigger in his fight than he did 2 weeks prior. 2 weeks! You DO NOT get substantially bigger in 2 weeks time, naturally.
Again, for the people that haven't worked out a day in their life and don't understand how hard it is to put on even a little muscle, I understand his muscle gain may look "normal" to you, but it isn't. Not even close.
No disrespect, but I don't think you understand on how muscle and size can be gained or lost. A man who is 6'5 with a long frame like his can easily gain 60lbs over 3 years and remain lean and muscular. 20lbs for a man built like that in one year ain't the same for a guy 5'10 gaining 20lbs. Second I would imagine quite a few of these guys carb load before a fight, which can lead to a person looking bigger beacuse they are holding water weight. I don't watch the MMA at all, so I don't know this guy from a can of paint, but looking at his frame, 60lbs over 3 years is certainly not over the top.