Local mail is twice the size as pop mail

What is compressing? Thunderbird has a compact method that cleans up the storage files and often that results in a space saving, but it does not nor is it intended to compress anything.

I mean compacting.

So there are duplicate messages in the folders to which you copied the pop messages?

The imap account is fine?

So there are duplicate messages in the folders to which you copied the pop messages? The imap account is fine?

The Imap account is fine, no duplicates. The old mailbox was when I used POP, so the pop mailbox was until 2016, after that I was starting with Imap.

I have looked into the folders on my harddrive.

Users\username\AppData\Roaming\Thunderbird\Profiles\default

from there to the Imapmail folder or the mail/local folders

The old pop-account had it's own folder, in there I could see what the size of the inbox file was, 300mb When I moved the mails to the local folders, the inbox file was 600mb. No extra mail was added, I just dragged the files IN Thunderbird from the pop account to the local account and after that to the Imap account. Somehow it wouldn't let me drag it from the POP to the IMAP, I could select a max of 10 mails before it started moving, after every 10 mails. I had to turn off Thunderbird, restart it, and I could move another 10 mails. When I was done, the moved folder with mails in my Imap account was also 600mb.

I tried to compact it but it stayed the same size.

So the conclusion is, that the old pop account was somehow 300mb, when moved to local folders it doubled in size, and again when moving to Imap it stayed the double size, no double mails no broken Imap.