I think you're looking at this from the wrong side.
It's not the size of a book that's fixed, it's the size of the ereader that's fixed.
If you have a thousand random books there will be a large percentage that are 'paperback sized', a few will be 'oversized paperbacks' , those ones that are always a pain on the shelves. But the rest are random sizes of anything from 'C' size to huge. For books the shelves are a problem, but there's not much downside to having a few different shelves for the wrong-sized books.
For an ereader you only have one screen size. As you've noticed it's usually about paperback sized because that's convenient for books that are just words; no pictures, no tables of numbers, just 5-12 words per line like paperbacks or newspapers. But as soon as a book starts adding pictures they're forcing a minimum page size, if that's larger than screen then you have a problem.
The answer is obvious, larger screens for the larger pictures ... but that makes the 'ereader' too big for a pocket, too expensive, not really an ereader any more.
Oddly enough the writers of the original series of Star Trek noted this problem in that there have always been at least two forms of the "PADD" or "hall pass", the handheld style and the clipboard style.
Looks like someone needs to make a "big screen" version of their ereader; identical (and so sharing the development budget) as possible to the "little screen" but sized A4ish. Or perhaps an A4 screen that you can attach the ereader to the back of.