Unstable backup size, where to look for causes
When using backup compression, the size of the backup will vary quite a lot assuming data is being changed in the database.  

Performing index maintenance each night will move data around inside the database a lot, resulting in unpredictable changes to the amount of allocated pages inside the database.  Index defragmentation is not perfect and does not guarantee the index is completely defragmented - it is merely a way of reducing fragmentation.

The amount of change you're seeing is nothing to be concerned about, especially when you see your file sizes are measured in kilobytes.