this,,,look at how many builds were ruined by extra weight,seo injections.like u said law of diminishing returns
Exactly. And as IroNat sad, they have reached their genetic limits.
Much better to just focus on conditioning once a pro card is achieved. I suppose over time, some bodybuilders have gotten bigger after turning pro - without completely destroying their lines and proportions - but by and large, I would say they are maxed out at that time.
Phil Heath made considerable improvements to his legs and back once he turned pro. So it does happen. But then you have pros like Art Atwood who turned pro in 2001 at 235-lb, and was competing at the 2003 Mr. Olympia contest at 285-lb, at great cost to the proportions he formally had.