His body is not used to carrying that - he can hit pads all day, spar, etc but when someone like Francis or Black Beast is coming at you and you're not used to carrying that weight (with those legs) you're going to gas fast as hell.
A while back someone on here posted him hitting pads and claiming he was looking sharp - he looked horrible, slow, plodding and off balance (the last clip he threw a kick and had to use the fence to keep his balance).
He looked slugging and slow in his last 2 LHW fights - that he lost in many eyes - putting on bulk is going to do him no favours unless he's hit tj dillashaw up for some EPO.