Except in the beginning he wasn't a monster in his calves and would pose with his legs in water even. Then one summer on an overseas sabbatical he came back with decent calves.
I agree it's never been proven.
However; he did finally admit to using/abusing steroids in the day...which many people still believe he has never admitted.
ive seen a few interviews where arnold talks pretty openly about using steroids. It doesn't seem there was this stigma about them back in those days like we have now.