From what I've heard, diaper manufacturers distinguish diaper sizes by waist size, but what they really mean is hip size. When you think about it, the "wings" of a diaper that wrap around the sides of your body to keep the diaper in place have to cover the widest area of your body too. If you have a 32" waist size but your hips are 35" wide, and you buy a diaper that only just barely fits 32", you won't be able to use that diaper simply because it won't fit over your hips.
With normal pants/jeans/trousers, the hip/waist size issue isn't that big of a deal because the pants bulge out right below the waist band and thus give you extra room for your hips. Pants are, indeed, held up at the top end, where the waist is, while diapers are held in place by the widest part of your body - the hips!
Peachy