Guys like to throw around big numbers, but then you ask them who in their camp uses and they say 'Well no one here does, but in the other camps everyone does!'.
I wouldn't be surprised if guys use them to recover from injuries faster or to help prep for a camp, but I don't think regular use is rampant, especially to get bigger. I think the Overeem cases of using them to get bigger are far and few because height dictates a lot in MMA. If you get bigger than your height than you end up like Sherk with T-rex arms compared to everyone in your division.