No he used lots of drugs to get big
You can't prove or disprove that statement.
However, take this into consideration.
-When Overeem was fighting at 205 lbs. he never lifted a weight in his life.
When he was fighting at 205 lbs. his "off season" or "walk around" weight was 235.
Here's a picture of him in 2004 at his "walk around weight" of of 235:
-By 2007 he decided to move to heavyweight (stop cutting weight)
His first fight was at 224 lbs. He admitted he had trouble keeping weight on during training and had to constantly eat calories because he was burning so many.
Overeem's eating habits themselves have made him
LEGENDARY. The guy eats constantly and in large quantities. You read about how bodybuilders and other athletes supposedly eat 10,000 calories/day, well I'd honestly believe Overeem consumes that much!
-It does not seem impossible for a young man to build 20-30 lbs. of muscle in a few years if he had never lifted weights before. He had great genetics as proof of his physique before he even lifted weights. Also remember, he only "works" a couple times per year. In his down time he can devote himself to lifting and training most of the year.
Overeem has added about 30-40 lbs since he walked around at 235 lbs. He's lean, but not "ripped" or dry. A lot of that weight was muscle, some of that weight is water.