after deleting messages the same free space on my usb drive

So, I have installed portableapps on my usb drive. After installing I have downloaded all my emails from gmail. After this I have created folders in "inbox" because of creating of the filters. After this I have deleted 90% of unnecessary emails, but the free space on the usb drive was the same as before deleting (???). The same space after restart. My USB is in my win system H:/. In H:/portableapps/thunderbirdportable/data/profile/mail/pop.gmail.com/inbox/.. was e.g. "friends.msf" (3kB) and "friends" (640MB)even though I have all the messages in the folder "friends" deleted. I had to delete the file "friends" to become more free space. And now the question: Why is not the file "friends" bigger/smaller automatically (bigger-new messages/smaller-deleting)??? (Another folders the same, I had to do this manual). The thunderbirdportable runs normal with no problems. Can anybody answer my question? THX a lot.