Have you compacted all your folders to make sure all 'marked as deleted' emails have been cleared to recover space? Do this first. File > Compact Folders.

Archives can hold quite a bit of data/emails. I have more in these folders than in my general use folders.

If you have old emails with attachments, this will increase the folder size. You can detach attachments and store them outside of Thunderbird, thus reducing the size of your thunderbird folders. I do this with documents or images I do not want to lose. For example, my son sent me an image of a scan - new baby expected soon. This file was 3.7MB, so increases my folder and Profile size. I want to keep this image, but reduce my Thunderbird folder size. right click on attachment and choose detach. Select where to save it maybe in one of the Windows auto files like 'Documents' or 'Pictures'. I can still select the attachment and see the image as a link is put in its place, but the email size is now only 6.5kb.

Make sure Thunderbird is closed when performing anything in Profile folders. The global search will also create a large file. It can be located in your Profile folder: global-messages-db.sqlite

You can delete this file. It may contain data which you have since deleted. it will be created again and when you search it will be repopulated with more current data.

Your junk.log file will also be of some size, but I would not do anything with this.

Thank you -- very helpful in part. I lways remove large files from emails so as not to clog the profiles. I removed Global-Messages-DB and compacted the files, as you suggest. It had a dramatic impact on the Profile called Roam -- it went from 228Mb to 140 Mb. But it did not change the Thunderbird Profile called 'Local' -- wjhich has a bunch of cache folders in it. How do I manage that Profile? Can it also be deleted and Thunderbird will re-generate? Thanks.

You can use a add-on importexporttools to export emails https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

That is a good prospect to try. But I have deleted half of my emails -- and the Roam Profile decreases proportionately -- but the Local Profile does not. What is actually being 'cached' in that Profile and why does it not decrease when I both compact emails as well as delete emails -- nothing I do seems to affect it? Thanks.

Your local folders are within roaming and there within mail Not in appdata\local

%AppData%\Roaming\Thunderbird\Profiles\?????????.default\Mail\Local Folders

If you want to get rid of all cache tools / clear recent history will do that

Much THANKS!! That does it. Went from 105 Mb to 2 Mb.