My search didn't find a previous mention of this, so I'm posting it in case it helps others.
My LR catalog has ~50K images and was about 7GB. From some web research, I learned:
- The develop history entries can take a lot of space
- The Delete History operation just deletes the steps taken to get to the current state of the image. That is, it doesn't revert the slider settings, adjustment brushes, etc. back to the default state.
- You can select a bunch of images (e.g. in Library), go to Develop, do Develop>Clear History. It will give you the option of clearing the history off all selected photos.
I deleted the Develop History of all my photos from previous years (about 47K). After optimizing the catalog, it's size was down to 2.2GB.