The typical ereader has several GB of non-volatile storage these days. LaTeX and AMSTeX seem to be about 200 MB, even though they provide a much more sophisticated typesetting and layout than ePub. That includes all of the fonts, including those for handling formal languages (like mathematics).

Now I'm not suggesting that TeX is a model that ereaders should aspire to. It is a bit inefficient for the current generation of ereaders, particularly since it uses multiple passes to render a document. Yet it also demonstrates that ebook standards have a long ways to go, and that they should aspire to something better than a glorified medium for novels (and certain classes of non-fiction).