I wouldn't be shocked if they dose as much or more than IFBB pros. The difference between these physique guys and pros are their responses to drugs. Heath off season is probably the same size as a physique competitor but once he sauces up he blows up 10 fold.
There were many pros like Chiuaha Ray, Milos, Darrem Charles, Troy Alves etc...who were super heavy users but genetics wouldn't let them get as big as a Levrone, Nasser or Yates