Mac storage basics · Finding big files, Downloads and the Trash
Why does Get Info show two sizes — the size and "on disk"?
Short answer: Size is the content total; on disk is the space actually used. They differ because of block allocation, compression, sparse files and online-only cloud files.
The first number is the total of the file's contents; on disk is the space it actually occupies. They differ because storage is allocated in blocks (many small files take more on disk than their contents), some files are compressed by the file system, sparse files reserve less than their size, and online-only cloud files take almost nothing on disk.
For freeing space, the on-disk figure is the one that matters.