Notwithstanding that Flex is hardly "skinny", he'd just started training. What did you expect him to turn into a monster in a year or two?
There's at least a DECADE'S difference between those two pictures. So, just ignore the eating, the training, the sleeping.....all the other stuff.
All Flex did was inject steroids and POOF!!! Instant monster!! 
Notice how nobody compares Dorian Yates' high school pics to his Olympia form and claim he's "all drugs" or sarcastically claim that "drugs are just the finishing touch".
a. That wasn't my point - I meant that he started off very skinny, was all - didn't criticise him in any way (or mention a time frame)
b. I
would say the same for Dorian actually - they are
all all drugs - again, not a criticism, I love drugs!
Y so salty, Bro? Need a hug?

Edit: how about this pic of Dorian when he was younger compared to his Olympia days..? "Just the finishing touch"
