Well in the latest interivew he says hes 234, he came in light for Stiverne but has been around 228 since then, so not a huge increase in weight.
That's not much weight to gain if he's been weight training seriously. A caloric surplus+serious weight training= weight gain. It's not rocket science.
That's only about a 10 pound weight gain give or take, he'll still look rail thin in the ring. It can easily be achieved through natural means.