This is the most predictable fight out there. Briggs has terrible stamina. Vitali's whole game is based around outworking his opponent and tiring them out.
How much more clearly can it be said? WAY to much muscle for boxing. It'll greatly diminish is performance. But, at least he'll look in intimidating for a few rounds.
Which is a feat in itself, given that Vitali is such a massive guy, and guys that big are usually known for weak stamina.
:deal If I was to sum up the reasons for Vitali's success in one sentence, I'd say "Big guy with stamina".