That's true, but does it matter? It depends on your final print size, your film and developer, and your tolerance for grain. I happily print up to 16x20 from my 645 negs, or 6x6 cropped to 645. The bigger negative would primarily be an advantage to me for cropping further for composition or to make up for a lens a bit shorter than I would have preferred.
If you print larger, use film that's grainier, or are averse to any visible grain at all then it starts to matter more. I can certainly see the difference between 645 and 6x7 on a 16x20 from fast film, at least if viewed fairly close. More commonly I print 11x14 and at that size the difference becomes very small.
I totally understand the appeal of the lack of grain and smooth tonality of a larger negative enlarged less - I do shoot 4x5. I'm not familiar with the GS, at least in person (never seen one IRL.) The RB is just bigger and heavier than I, or many people, want to carry and use handheld so if the choice is between the RB and Hassleblad it comes down to that. Now if I'm going to use it on a tripod all the time, I'd take the RB hands down for that bigger negative. I'm not a believer in the church of Zeiss or any other magic lens make. Good lenses, yes, special magic in certain brands, no. Mamiya glass is excellent.