I don't know what you mean by setting up for online storage, so I will assume you meant the folders were set for offline access an to sync 100% of the messages. Here are some possibilities: - click the synchronization pane - click 'advanced' button and untick all folders - change sync settings from 100% to a smaller number - also tick the box for messages above a certain size (make it small)

Since this is an IMAP account, you could also - exit thunderbird - locate the files (e.g. inbox, inbox.msf) in profile and delete them (or, if you are unsure, MOVE to another location) - tip: while there, look for any NSTMP files as they can be deleted. Look also for duplicate account setups (account names followed by -1, -2, etc.) as the NSTMP files can be immediately deleted, and the others have a possibility of having duplicates of your messages. If found you would need to review. - restart thunderbird and the folders will be repopulated.

Now, you can see whatever the savings are. This is a difficult assignment, because you still need headers of all messages so you can access as desired.

Thanks.