The mdbx granularity seems to be one megabyte, so it will only grow by one mb when you "save" it, if it runs out of space @ current size. I noticed sometimes it takes a fairly long time before it grows by one megabyte; I feel as though the growth has gotten slower despite talking about as much.
If you are worried about your data, you can conduct a simple test - install MsgExport plugin and export all of your contact as json.
Move the files somewhere for comparison and then use miranda for a bit; then export the data again and compare the contents. You should see that in the new exports, the new messages you wrote should be present. If not, or if there is some other discrepancy, there may be something wrong with your database.
You can also take a complete backup of your miranda and then create a clean profile and import your current profile with all of its setup into the new clean profile and see how big it is then.
I also recommend having the MsgExport plugin configured and using it on a daily basis because you never know when mdbx decides your data is not worthy of being stored...
There was also a limit for db size on 32bit miranda with mdbx database, but you are nowhere near close to that number so it should be ok (if the limit is still even there)
