'compress account' not working

Hi guys.
First of all: thank you for the app! Great job.

Am using K9 for many years, on 3 different Samsung phones (S4, S5, S8)
On all these phones the effect is the same: I keep a clean folder/account history (deleting mails from waste-bin etc.) My K9 should be small always.

But whenever I select ‘compress account’ (Konto komprimieren) - the effect is… 0
t keeps bloating and bloating, right now I am at 200 MB for hot air. The only workaround so far was to re-install and that is not exactly satisfactory…

Am I doing something wrong?