I think that would be pretty hard to define as there seem to be a lot of brands (mostly Generic/Store Brand, meaning somebody else is making them), and different brands have different measurements and a slightly different fit.
I will say though that a Size 7 can fit much bigger than the listed size, mainly due to how big, tough, and stretchy the material of the Tabs can be.
If I had a 6 inch smaller waste I could make a Dollar General size 7 fit on me without a second set of tabs, but I don't know if the tabs on other side 7s are as good, they seem strecthier on Pampers, but Pampers has been shrinking their diapers (they may still fit the size they say, but the ones from like 5 or 6 years ago had a bigger area of padding, were more absorbent, and were just bigger overall).
I would say if you are a smaller sized adult, like say 28 inch waist you would probably be able to fit in most size 7 Baby Diapers, but they don't really compare to a good ABDL Diaper brand (if they have a size Small that fits you, which as stated earlier would depend on Brand/Model)