Let me clarify my original premise. I'm talking about the difference between one inch increments in ski size up or down if and only if you are on the bubble to begin with. Bigger swings in ski size are going to have much greater and likely unfortunate effects.
I believe he rides a 66, which at 36 mph makes sense for his weight. From my perspective, that's absolutely normal.
(Actually the last time I skied with him, he was on the 6,6 and I think that's what he still rides, but I wouldn't be shocked if he'd move up to 67)
Yes, between a a 64 and a 70 you would be correct but a one size smaller ski is slightly narrower and going to roll over further. Even though it's going to take more strength, you're going to have more angle therefore more cross course speed. You might not call it faster if your definition of faster is ease of speed creation.