The weight limit is based on total body weight. At recommended hydration levels, a certain body weight will produce a certain amount of pee. The baby diapers are made to adequately contain that amount of pee in most any situation.
A baby diaper rated for 35 lbs is not capable of adequately holding the pee of someone who is 70 lbs- regardless of physical fit.
This is why baby diapers do not work for adults. Now I get having adult diaper sizes based on physical waist size is backwards, but society as a whole seems to have fallen for the old "well we can just change more often bs". This is what should really bother you.