Overeem fought at HW a couple times weighing in at 220-225 range
Against Paul B it was 11/2007 and he was 225 lbs
Against Cro Cop in 9/2008, 10 months later he was 242 lbs, his next big jump in weight was against James Thompson at 10/2009 where he was around 255 lbs which he has been pretty much since than
So he went from 225 to 256, 31 lbs in 2 years
He has said in the past he rarely if ever weight trained to stay in the LHW class, he didnt start lifting until he decided to go to HW, Overeem gassed badly in almost all his fights at LHW when they wouldnt end in the 1 round, he clearly wasnt a LHW and a natural HW, he has a big frame