In my experience, text is more compact in MOBI than it usually is in ePub resulting in smaller files if, and only if, the ebook has only a minimal cover and includes a lot of text (novel +).
On the other hand, images don't seem to compress/compact as well in MOBI, at least when calibre does the ePub to MOBI conversion. The difference is largest when the images in the ePub are GIFs. When I build illustrated ebooks with a number of images to ePubs are usually smaller.
I recently built both ePub and MOBI version of
The Rome Express by Griffiths that include a decent sized cover, one frontis JPEG image and two small images (GIF) on the title page along with an ornament (GIF) at the beginning of each chapter. The ePub is 248k and the MOBI is only slightly larger at 313k.
A similar pair that include a number of nearly full screen JPEG images,
Three Men in a Boat by Jerome, have a much greater difference. The ePub is 1508k while the MOBI is over twice as large at 3462k.
A third example is
Lysistrata by Aristophanes. Here, all of the illustrations, except the cover, are GIFs in the ePub. The MOBI version is nearly 3x the size of the ePub, 632k vs, 1853k.
You can download these books from my
Lakemere Press site if you want to examine them.