No way in hell those numbers compute. Dexter doesn't weight that much.
Whatever he weighs, he's the best in the world, period. The point, of course, was that there is no big difference in weight between Jackson and Schwarzenegger, or Jackson and Haney ('91) or, Jackson and Yates ('91 and '92).
Even if Jackson were 225, that would be just as massive on his body as the 240 on Arnold's 6'2" frame.