Originally Posted by Corpsegoddess
They're generally the kepubs I buy from Kobo--I've noticed those tend to be on the larger side.
Kepub is not epub. No matter how similar the names sound. Keypubs contain database-like elements for all kinds of extra "features." If file-size is that much of a concern, then re-download the books from Kobo in the standard epub format instead of their proprietary keypub format. I think you'll find the size more like what you would expect, based on content.