I believe 'size on disk' shows how much that folder is actually using in stored emails - it is not a limit preventing any more from being stored. So - when there are messages in an folder the size shown will grow. If you drag or copy a message from Trash to Inbox or to any other empty folder you should see that. Why are they all in trash? Have you set up a filter that is to take an action on incoming emails and then delete them? That would put messages in Trash. That could be happening either in your online webmail - messages sent to Trash before they even get to Thunderbird, or may happen after they download if Thunderbird is moving / deleting them. Shut down Thunderbird, then check your webmail to see if any incoming emails do stay in Inbox there. Once you have a few in there, then start Thunderbird and see if they go to Trash or not. Please report the results.