My mac recently started to indicate that the startup disk was full. I went thru and cleaned out all the disk images and videos and other large files that I could find. Emptied trash. Emptied downloads.
- 2012 MacBookPro with 1TB SSD
- macOS 10.14.3 with 4 users (me primarily, wife and two kids who rarely use it)
- Virtualized with Windows7 via VMWare Extreme
- iTunes is not used, no music
- iMovie: cleaned library
- iPhoto and Photos still installed
A suspicion: From what I've read about when Photos replaced iPhoto that used a pointers system for Photos to point at the iPhoto files from the old iPhoto library rather than creating a new duplicate library. I still use iPhoto for my photography hobby as I can color correct, edit, organize, etc. Photos was imposed on me in a software update.
Anyway, if you look below, the two outer rings are both about the same size. The outermost ring has the same name as the item at the top of the list on the top right. The next ring has a different name and is slightly smaller. The "storage" button (screen shot above) from "About This Mac" lists a much smaller amount counted as photos (yellow).
Last is the Photos info button results (iPhoto doesnt seem to offer info about the library linked to it)