'Thunderbird duplicates Gmail structure?

I have a GMail account, from which I download my email in Thunderbird (in Linux), and keep it ordered locally.

I noticed that my backups are very slow, because Thunderbird has two big directories that get updated often:

11G /username/.thunderbird/k0aa999s.default/ImapMail
7.0G /username/.thunderbird/k0aa999s.default/Mail

under /username/.thunderbird/k0aa999s.default/Mail I have my local folder structure, but what is under ImapMail ?
I guess it is the corresponding email structure I have on the gmail server, but gmail reports that I'm using 5.67GB, not 11GB.
My 11GB are created by just 4 files:

1.6G Apr 22 09:55 Sent Mail
3.0G Apr 22 14:16 Trash
2.6G Apr 22 14:16 Important
4.3G Apr 22 14:20 All Mail

I saw it suggested to rebuild the folders in Thunderbird - I did it but nothing changed

What can I do? I dont need / want to replicate Gmail structure locally, I just want (and use) only what I have under /username/.thunderbird/k0aa999s.default/Mail



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