Unwritten pages in files that are extended don't actually take up space or appear as blocks; a good backup program should respect that. See https://en.wikipedia.org/wiki/Sparse_file
Indeed, but many programs, backup or otherwise, or unaware of it. IIRC gzip/bzip/zstd don’t (will spend time reading and writing all the missing pages) whereS gnu tar does.
Zstd by default will write sparse files when it detects runs of zeros during decompression, but isn't aware of it when reading a file.