Scientifically speaking here...
Humans can only get so big...
1.Carrying over 300lbs shredded at 5'8" takes a toll on the bones.
2.Carrying over 300lbs shredded at 5'8"takes a toll on the internal organs espically the heart.
3.Doing what it takes to get to 300lbs shredded at 5'8" takes a toll on the whole body.
4.Skin can only stretch so much. Guys trying to get up to 25 inch biceps(Which is what we'll see in the future if this rate of growth of bodybuilders continues) will start seeing problems with their skin.
A 5'8"-5'10" frame can only carry so much weight. Considering Bodybuilders aren't getting taller..They'll start seeing alot of problems.
Unless some new technology is invented in the future to completly alter their genetics to make their bones stronger,Skin more elastic,Heart stronger and make them taller..I simply don't see the growth of bodybuilder size continuing much longer.