I have bunch of ebooks from Packt Publishing, that are delivered in formats PDF + EPUB + MOBI.
MOBI is usually Joint MOBI/KF8s and is larger than EPUB. However after split a resulting AZW3 and MOBI are far smaller than joint, and than EPUB.
In one particular case Joint is ca. 14 MB, EPUB is ca. 9,7 MB and resulting AZW3, MOBI are about 3,7 MB each.
Resulting AZW3 and MOBI are apparently valid, they look identically in Calibre viewer (and identically as Joint, which in turn is inferior to EPUB, e.g. Joint lacks of colors in source codes; there is no problem in AZW3 after conversion from EPUB).
Such huge difference in lengths of source Joint and lengths of AZW3, MOBI (combined) is inexplicable for me. If someone competent would be willing to check this issue I can send him via PM a link to archive with source EPUB and Joint MOBI/KF8. Free ebooks (different title daily) are also available:
https://www.packtpub.com/packt/offers/free-learning
after registration (but Joint MOBI is not always available for given title).