Arnie is/was actually a lot smaller than most people think. Skeletally, he's somewhat similar to today's pros. But he also carried less size overall.

Plus, he was somewhat thick waisted himself. If you notice, in almost all of his front double bi poses, he's twisted his hips and waist to give the illusion of a smaller waist. That was more of a norm in that pose back then, but he almost ALWAYS did it.