A lot of bodybuilding is an optical illusion. It is more about the appearance of size rather than the actual size.

In that first pic he was probably about 20-25% bodyfat and the after he was probably under 10%. That makes a huge difference in how big you LOOK.

A great example of this illusion is Frank Zane. He competed at about 175-180lb. He was the lightest Mr. O ever. Even though I outweigh him by 20 lbs, he looks a hell of a lot bigger.