Model database backup size grows

Good morning,

We take nightly backups of both our production data and model databases. The model DB backup is precautionary as we don't touch production outside of a code promotion. However, I've seen our model backup file (compressed) size grow ever so slightly (1K to 2K) with each backup. I understand the model database to hold all the code which run AX, so if we're not changing that code outside of a code promotion then why is the BAK file growing? I'm sure someone out there can enlighten me on this topic. 

Thanks in advance.