How to bring a Time Machine backup to its knees: get it to back up a sparse file to an HFS+ disk. It’ll take forever and run you out of free space.
Sparsity
Looking in more detail at newer tricks used by APFS on the data of files: sparse files, which can squeeze vast empty files almost to nothing; file clones; and compression, opening up in Big Sur.
An ingenious idea to save storage space: leave out empty areas in large files – make them sparse. Do they exist in APFS as claimed?
The Eclectic Light Company 