It's called lifting weights and strongman training. If you watch the countdown to UFC video before the 2nd Brock fight, he states that he never took weight training seriously up until that point.
so gaining 50lbs of muscle while cutting is just an affair of lifting some weight, walking with stone and pushing tree trunk, especially when you're a guy over 30 y.o...
been lifting weight since 97', got a huge frame, i weighed 300 lbs but while staying natural i can't go over 235 if i want to stay in a descent shape, and i train 5 day a week (2 hours each), plus cardio and boxing twice a week on rest day.