According to my email provider, my mailbox is 5.2 GB in size. However, according to the Disk Usage Analyser, Thunderbird uses 9.6 GB.
I use Thunderbird with this my email provider (posteo.de) in a debian vm. But the problem also occurs in a fedora vm. Interestingly, the problem does not occur with another email provider (in this case also Thunderbird in a fedora vm).
Any ideas?