For what it is worth, it is getting on for 15 years since I last had centre sheeting, and that was a free-running block with a jammer, rather than a ratchet block. But 75 mm is 3 inches in old money, which seems fairly enormous; I would expect 57 mm to be more usual, and just looking at 57 mm on a ruler it seems compatible with the sort of size that I remember.
Arguably the rope will run more freely round the larger sheave, but I was never conscious of any impediment with the one I had.
Oliver