MySQL IDB File grows huge despite having little data
Some 30 hours now have past and the table in question (i.e. its idb file) has less disk space occupied than expected. The application that uses the table had been offline for a couple of hours, too, and hence, the table has been shrunk again. It still shows its growing behaviour once the application is coming back online again.

It appears now, that there is a purge system within mysql that does not quite keep up with usage of that table - despite the older 5.7 MySQL on much older hardware didn't have that issue.

I suppose that this realization is the answer to my original question and that I will have to tweak the innoDB purge parameters to find optimal settings, or maybe trigger Optimize Table from the Application every once in a while.