What should be in a MOBI file?

This a bit of an odd question, but my experience of MOBI is a bit limited and I wasn’t 100% sure if this file was as strange as it seems.

From the quality of their ePUB, I’m guessing that this new publisher doesn’t quite know what he’s doing, but anyway here’s what’s Calibre’s KindleUpack gives me...

\mobi7\
\mobi8\
kindlegenbuild.log
kindlegensrc.zip

Is it normal to include this source file in the MOBI? It’s 4MB of the files’ ~5MB.

The ePUB edition is ~3.5MB because it has a huge resolution cover (3.2MB vs. 111KB in MOBI edition). There doesn’t seem to be a very coherent strategy here.