O Thunderbird usa arquivos grandes para armazenar várias mensagens. Ao deletar, somente marca a mensagem como deletada. Você precisa compactar as pastas para recuperar o espaço usado pelas mensagens excluídas:
The easiest way to compact is to let Thunderbird do it automatically: "Tools -> Options -> Advanced -> Network & Disk Space -> Disk Space -> Compact folder when it will save over 100 kB -> OK."
Ou:
To compact all folders in an account manually, click the account on the left, and then click "File -> Compact Folders". This has to be repeated for each account. Compacting an account may take from a few seconds to 10 minutes or more, depending on how much mail you have and how recently you last compacted the folders. If you have trouble doing this and the process stalls, try compacting one folder at a time by right-clicking on the folder and choosing "Compact" ("Compact This Folder" in older versions). If you do not let Thunderbird compact automatically, you should do this regularly, at least once a week. If you do it daily, it will take only a fraction of the time.
Mas, enquanto isso, o arquivo de mensagens ficou enorme: tenha cuidado. Se possível, primeiro tente fazer o backup de sua pasta de perfil e antes de começar: leia as outras instruções .