Converting (Kobo) epubs to reduce file size?

Does anyone know of any method to reduce the bloated filesizes of Kobo epubs? I'm talking 2-3 MB per book (from Kobo), as opposed 250-350 KB (From other sources, such as OmniLit). I was thinking the Convert Book option would do it, but although I've been able to fix some of the other issues (ie, fonts that grow and shrink every couple of paragraphs), nothing I've done so far has significantly changed the file size.

Please help!