InDesign CS5.5 to Epub: Newly Bloated File?

Question

I have an InDesign EBook File consisting of 25 chapters and many maps and photos, and up until now everything was hunky dory.
I'm using BookBaby as an aggregator because ISBN's are so much cheaper with them. I sent the preflight-checked, Caliber-passed Epub file to them and they bounced it back for a few problems. To comply with their requests, I changed all the InD file names to eliminate spaces, substituting_underscores. I had to redo the Table of Contents in the frontmatter, embedding hyperlinks to each of the book's chapter files (somehow my earlier work on this dropped off the file before I originally sent it to BookBaby).
Done with their requests, I doublechecked all the image files and doublechecked the inline placement of those scores of image files. To make sure that each InDesign file was as streamlined as it could be, as I've done earlier, I went into each of the 25 ID chapters to "Save As" with the same name, which should have eliminated everything nonessential. The InDesign book document then saved as a whole, I ran preflight pub checks several times, fixing what was necessary, until I got the row of green lights.
Then from ID I exported as an EPub. Using Caliber as my reader, everything checks. But to my horror, I noticed that the EPub file size had bloated from the 16 mb I had sent BookBaby originally to 54 mb!!! Obviously, I've done something terrible. I've been trying all day to figure this out and am despairing. The epub seems otherwise identical to the earlier, albeit flawed, version. Granted, I have a _lot_ of images. But why did I get by with 16 mb overall epub filesize, and why do I now have 54 mb? Perhaps it was in the 25 maneuvers I did to "Save As" each chapter file?

Answer 1

Many thanks, Peter. It's helpful to get any new knowledge, even if I'm over my head. I'll continue my inquiry, especially into the two areas you mention, and hope for more insights from other contributors!