You are correct; I too hastily took the "payload" in the tar-split.json.gz to be the binary itself, but it's something else.

Nevertheles, an archive format other than .tar.gz could be transferred more efficiently given a previous version of the same image. Or, even .tar.gz could be transferred more efficiently if both ends extract the file first, and if there is a need to preseve sha256, then that needs a bit more work.