Mac storage basics · Finding big files, Downloads and the Trash
Why doesn't copying a big file use up more space on my Mac?
Short answer: APFS clones a file copied within the same drive, so both share the same data until one changes. Deleting one of a pair of clones frees little space.
On APFS, copying a file within the same volume in Finder makes a clone: the copy shares the original's data on disk until one of them is changed. That's why the copy appears instantly and free space barely moves.
The side effect is that sizes can mislead. Each copy shows its full size, but deleting one clone frees little space while the other still exists. Only the parts that differ between them take extra room. Keep this in mind when removing duplicates — see duplicate files.