The reason some people have this problem of extra backsheet material on the top and bottom is because it's very expensive to make disposable diapers in more than a few sizes. To add more sizes in sewn garments is quite easy to do, but for disposable products it could cost millions of dollars to support an extra size including all the downtime on machines that cost $25MM to change sizes more often...which they desperately try to avoid.
With the given sizes, you need to have the length (aka height, aka rise) of the diaper be tall enough for the largest people that would use that size diaper...otherwise, for too many people, it will not come up tall enough in the rear to cover everything. It's a business decision as "too tall" for some is far better than "too short" for some. As you've seen, people can adjust if it's too tall...but can't adjust if it's too short.
There is a "Regular" size in some brands that is this purpose. It has the width of a large but the height of a medium. However, as companies have been investing in newer, faster, much more expensive machines, most have made the decision to cut the Regular size so they could keep the machines running more and reduce the changeovers.