Yeah, good point. As it says in that photo he weighed 195 and he looks around 7% bodyfat. When he was 210 on stage he was 5% I'd say, so that's about 20 pounds of lean dry muscle added which is quite a lot actually. But then again, he was only 19 in that 195 pound photo so he would have probably added another 10 pounds just from maturing, and if so that means the gear only added another 10 pounds or so to his frame.
Yeah.
Some Trivia. There was a video posted here a while back of a then young guy who trained with Mentzer for a bit. The guy had to drive Mentzer to a meth dealer before he would go to a gym lol. Anyway, he said him and Mentzer would take 3ccs of NPP 3 times a week, nothing else. So I think Mentzer had taken a bit more at times than what you quoted. Another poster on a different forum said Platz told him to take 200mg of Deca day.
In Muscular Development, shortly before Mentzer died, he said he didn't think the then current bodybuilders got anything more out of their megadosing except larger quadriceps. That was the only improvement he saw. I don't agree really but it's an interesting observation. Then you factor in the GH and insulin so how much did the increased steroids contribute? At the same time, I don't think you have quite exhausted the potential of steroids at 1500mg but it's not a linear muscle increase as the doses go up, obviously. Maybe you get to 95% at 1500mg. Victor Conte who tested all the top pros in the 90s said they were all at 3-5 grams.
I think Mentzer could have put on those extra 10-20lbs in say 6 months with a drug increase. It tends to come pretty quickly as the stimulus is increased. So that's why I say he was close even at 19. Same with Casey Viator, he was a monster at 19, and arguably looked his best ever at that age already.