Which file(s) in your profile is/are the large one(s) then?

I don't think it makes much sense to subscribe to 'All Mail'. So if you don't really need it, you could simply unsubscribe from 'All Mail'.

When did you compact folders the last time?

All Mail is the huge folder. But I only see Subscribe as an option on the File drop-down menu. I'm using TB to access my Gmail account. Do I unsubscribe from All Mail in Gmail rather than in TB or in both?

I can't compact folders because I don't have enough hard drive space and it aborts when trying to compact the All Mail folder. And even when I could, it only freed up a couple of GB.

Also, when I launched TB today, each paragraph in all my existing emails changed to a mix of different sizes within the same email. This has happened before and when I restarted TB, everything went back to normal but that didn't work today.

Unsubscribing from All Mail worked -- it freed up 26GB of hard drive space!

Now my only issue is the mixed font sizes in all my existing emails that were originally one size throughout.

rebooted your computer since freeing space?

Doesn't help the mixed font size problem.

I could understand that one folder being corrupt and doing weird things, but I think your saying multiple folders are affected, so I am really just working through what I think might work..

Take a backup of your profile (I am a bit concerned corruption may see mail disappear)

Then using compact on the file menu compact all of your folders.

Exclude the Thunderbird profile folders from "on access" or "resident" scanning in your anti virus.