Thank you for the quick answer.
As already described in the article above ( 'compress account' not working ) - despite complete deletion of the mails - the respective account gets bigger and bigger over time - “blown up”, so to speak. Therefore there is in the version 5.600 the possibility to COMPRESS the desired account together with its folders: → Account → Folder → 3dot Menu (the three points right down) → Compress account.
I can’t find this function in the latest version of K9 so far.
Where can I find this function? Or does it not exist anymore?
In the corresponding mail program “Thunderbird” for Windows and Linux there is also such a function: → File → Account → Compact Folders
Considering there is only one payed developer, the rest of the people here are users like you. They ask questions and some volunteer their spare time to answer other users’ questions. No obligations unless you are willing to pay for support staff.
I don’t thing the compression option survived beyond version 5.600. You could purge messages and then re-synchronise the account. The effect is the same.
Thanks for the news and information! If I delete all local messages regularly, the size of the app on my smartphone is fine. In this respect I think that my problem is solved, for which I would like to thank you very much.