It's because I've switched from "Mobi Creator" to "Kindlegen" to create the Mobi version, and my understanding is that Kindlegen inserts a copy of the complete HTML source into the file, roughly doubling its size. I don't know why it does this!
The ePub version has increased in size from 2.7 to 3.1MB for this version, due to the improved illustrations.
I'm sure you'll like the fact that it's now using "proper" paragraph indentations, rather than hard spaces
.