I use Thunderbird with few accounts. One runs Dovecot, two are on GMail. Half of my desktop SSD is used by ~/.thunderbird/ folder nowadays.
Decided to take a look why.
Went to biggest folder (1.8GB in 57900 files) and then checked how much Thunderbird thinks it takes - 1801 messages, 48.9MB in total. Each message is stored over THIRTY times. Maildir format, GMail account.
Went to other folder. 823MB in 77345 files on disk. Thunderbird says 3107 messages, 30.7MB in total. Maildir format again, same GMail account.
Ok, let's check Dovecot one. Turns out that this one is mbox based. Biggest one was 2.1GB on disk, 526MB according to Thunderbird.
For each of them I went with "repair folder" button which is basically "drop whatever is on disk and fetch again from server".
So today instead of working I will go folder by folder to 'repair' them to regain my storage space. And then add to the calendar "go and repair all Thunderbird folders" monthly ;(