That comparison really shows how far Arnold came in terms of upper leg development. He is absolutely destroying Sergo there in upper legs.
No, that's way off, if anything thighs were Arnold's biggest weakness even in his greatest shape. What Arnold did everything possible for his lower body to compensate for the thigh size deficiency in both quads and hams. Great cuts, aesthetics and sweep coupled with great calves and posing that always presented them at the right angle. All those things helped to take the emphasis off the deficiency in overall thigh size.
Quad size was much better on Sergio, and in shape Sergio also had solid cuts there, along with very good calves and hamstrings. When in shape, Sergio's legs were just about perfect, virtually flawless. When in shape, Arnold's were pretty damn good but still somewhat flawed in terms of thigh size.
Therefore, at best even with the cuts Arnold's legs would've been a draw against Sergio. That was his intent i'm sure, to try to reduce Sergio's lower body advantage, which was significant.