Thank you for your responsses! So if I understand correctly, the file size is bloating up to 40MB because (1) my CSS has to get merged into the HTML and (2) all the tiny PNGs get blown up into JPGs? Furthermore, the file size reduction I did on my PNGs would have no effect, because MOBI does not support PNG? I guess this makes sense.
Makes me hate the MOBI format all the more. I just wish Kindle would get with the program and work with EPUB a bit more nicely! Bloating a 10MB file all the way up to 40MB is really killing this.
I also tried converting the EPUB using KindleGen. It was my first time using it, as I generally prefer a GUI. The result is 25 MB, which is certainly an improvement, but it did a poorer job preserving formatting than Calibre usually does. I will continue to use Calibre for most books, but it's unfortunate that it has made a mess of this one. It's also unfortunate that the Mobi format is so much harder to work with than EPUB; I'm constantly unzipping, tweaking, and rezipping my EPUBs until I get them just right; wish I could more easily do that with my Mobis, too.
I'll send both the 40 and 25 MB files to the publisher and let them choose which one to use. Thanks for your advice!