The answer to your question, kind of.
The difference in length of medium to large diapers is much greater than large to extra large. The reason for this is because the diaper is supposed to fit around the narrowest part of your waist/hips to be able to stay up just like a pair of pants.
Smaller people need a smaller rise but at a certain point with larger people a higher rise just wouldn't make sense for one reason.
If your belly is bigger than your hips and the diaper were secured in the same place as someone with a flat belly then you would have issues with it falling down while wearing it unless you had a bunch of extra material that extended it above your belly button. When your belly hangs over your hip line the narrowest part of your hip area starts to get lower than that of thinner people.
Pants designed to go up high enough are fairly hard to come by and can be fairly expensive. If a diaper had all that extra material it would be out of most of our price range.