Yes, but what are the margins???
I mean, we´re talking about a 4" refractor in comparison to a C14? or are we talking about a 6" vs 8" scope?
What you say, can be true between small differences in aperture, but not between important ones.
It depends a lot on the scopes. A Ritchey-Chretien with a big central obstruction will have to be bigger than a Newton for it to beat a refractor. And a mass-market SCT with 1/4 lambda spherical abberation will have to be bigger than one with 1/10 lambda.
Apparently the heuristic of subtracting the central obstruction from the objective diameter works quite well.