My thought process on this is adding in the fit of 1-6 on the size people they are designed for. They are supposed to fit snug around the waist but there is a gap through the middle. Sometimes considerable when they are in between sizes and need to size up. This gap of air space is a part of why these diapers perform so well.
The size 9 may "fit" an adult XS but you can also squeeze an average sized 4 yr old in a size 5 diaper. For proper fit a 4 yr old would need a size 6. An XS adult would need a size 10. I took an image and made an outline for reference.
View attachment 133778This was a toddler somewhere between two and three.
The black line represents the silhouette of the body, blue is the diaper and red is the elastics. The elastics play an important roll in the creation of the air gap represented in... umm... we'll just call it cyan. They add to the bunching and folding of the padding that pushes it away form the skin. This is a bit exaggerated of course likely because this diaper was a bit on the big side for this particular wearer but leads to the next part.
This modification I did of a BetterDry is borne from a size medium. The wings are still the same width as they were when it came out of the package.
As you can see I'm pushing the limits of the diaper for a proper fit and my waist is only 32 inches (81cm) and my hips are 36 (91cm). Being an actual size medium adult this diaper should be just a bit bigger than it is. I would put this one between a small and medium leaning more towards small.
Based on size charts for kids, current diaper sizes for babies and toddlers, adult size charts and proper fit as described above I see a size chart looking something like this:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
1 to 6 M | 6 to 12 M | 12 to 24 M | 2T - 3T | 3T - 4T | 4T - 5T (4/5 XS) | 6 - 6X (S) | 7-8 (M) | 10 - 12 (L) | 14 - 16 (XL)
Adult XS |