I’m sure that someone with in-depth knowledge can wade in but the database will not just be storing the data but an amount of other data about the database, collections, indexes and a lot of other information so there will be an overhead to start with.
If you want to check in more detail I’d suggest putting more and more data in and then exporting it and comparing each to see what the overhead is.
I suspect it’ll be a certain amount of storage to start with and then as you insert more data the storage usage will grow according to the actual data size as opposed to growing based on the difference you’ve seen.
Bear in mind the server will also support compression (not sure if the 500MB limit includes compression or raw data size…)

So you’d end up with something like this:

1 Row: 1.1MB
1000 Row: 1.2MB
2000 Row: 1.3MB
3000 Row: 1.4MB

etc, so there is a basic overhead but after that the storage size grows as expected.

I’m away from my home setup at the moment so cannot test but give it a whirl and see what numbers pop out.