I checked on the Bulgari before you posted - the thinnest is about 5.25mm but most of them are 6.95 or thicker.
Certainly with thin watches, the larger the diameter the more they feel like a dinner plate strapped to your wrist. There are several Ultra-Thin models I've tried that are too large.
I think maybe Piaget is the winner for thinner across their product lines. This "Ultimate" is only 2mm thick! but it's 41mm in diameter.