There are a lot of contributing factors to that fight as to why Black Beast gassed even when they didn't look to do much. Mark Hunt does a lot of feints, he changes up his rhythm quite a bit and he was cutting off Lewis almost the entire fight. All these make you work and make you exert a considerable amount of energy. Hunt never allowed Lewis to set and always kept him on the edge. Hunt was trying to cut him off and back him up the octagon, and in turn Lewis would move and change directions every time he was nearing the cage, this also is tiring.
The whole fight was intense and Hunt's pressure seemed to have kept that tension tight the entire fight, again, that's another contributing factor. Top that all of with the strikes Hunt landed on Lewis and you can start to realize it's less about Lewis cardio (Although it is a part of that as well) that made him gas, but more about Hunt's game and how he was able to implement it and how deceptively tiring that fight was.
