It's a common mixup. Look at most any of the diaper manufacturer's sizing recommendations, and they will list your waist measurement. For comparison, also look at baby diaper sizing, they all recommend a waist measurement as well, with the only exception being newborns who should have their diapers extending up to below their belly button (though still over the hips).
Realistically speaking, a diaper should not extend only up to your hips/belt line. Especially for guys, as there needs to be enough absorbent material up front for the diaper to not leak over the top. Unfortunately you will hear enough from others who "feel" the diaper isn't supposed to be this way, and that of they do then they feel like they're swimming in a diaper that's too big. Well, that's what they are supposed to feel like, they aren't tighty whities after all.