The Klitschko brothers aren't just good because they are big, they are good because they are big AND good. Those two traits are not mutually exclusive. The other big guys around now are simply not good. There may be smaller guys around now who are pretty good, but all other things even, the good bigger man beats the good smaller man.

Now a lot of people like to hate on Mike Tyson because he lost certain fights, but I think it is important to remember that Mike Tyson was 5'11". That is small! Yes, he lost to Holyfield who, although formerly a cruiser weight, seemed to carry the extra weight into the HW division well. You can see that Holyfield makes Tyson look short!

I am just saying that for his size, Tyson was a good small man who eventually lost to bigger men.
Now Tyson Fury, Demintriko, and Helenius are big... but they are not that good... yet.
But it should be noted that the mere fact they are big already gives them the advantage against some of their smaller fighters who may have equal skill.

I assume all this should be obvious but sometimes it seems that some people over look the size advantage. I mean hell, we all know Adamek is a good, tough little fighter, but look at what his lack of size did to him against big men who were actually not good. Sure he won, but the big guy was always in the fight while poor Adamek had to give 100% just to tackle guys so much bigger.

It goes without saying that Vitali would be too much for him.