Images linked from files on your disk are never shrunk or overwritten. Shrink Big Images replaces packed and generated images with smaller versions, so keep a copy of the originals if you may need full resolution later. Merge, Shrink and Purge can be undone with Ctrl+Z.

Blender never saves unused data to disk, so it only costs memory. Purge frees that memory and tidies the file. To shrink the file, use the other fixes.

Packed file sizes are exact. Mesh and animation sizes are close estimates; in our test the total was within 1% of the real file size.