He only gained 1 or 2 kilos but that's because he exchanged fat for muscle.
Fat is the most efficient reserve of energy a body can have,
read this if you want to gwt in to it a bit better:
Lyoto Machida, former UFC LHW hamp
Future UFC MW Champ
Benavidez, Barao, Aldo, Nate, Nick, Lyoto, TBD, JDS
Forever hanging in Anderson Silva nuts