It'd also be quite hard to do, and I think generally speaking we prefer it this way.
It drives me nuts when in Nickel it's suddenly not responding because it's loading the next chapter, I press next page again, and then suddenly it turns two pages.
Only that means a gigantic omnibus should be split up a bit into different files.
Size is fairly meaningless by itself btw, if most of it is images for example or even just plain text. It's mainly if you have huge numbers of HTML nodes that performance/memory can be an issue.