That's a Microsoft link, but you may be on to something: "sparse files" exist on ETX4 too. It looks like that's what Electrum is doing, there must be a lot of empty space in the file. That brings the next question: why not just delete the empty space?
It is indeed a sparse file. Electrum actually has a function that marks it as such in their util.py: https://github.com/spesmilo/electrum/blob/dfa0dd47b728d22a3b8143af4bdd1819a0997abd/electrum/util.py#L490.A good bulk of the headers file consists of zeros, and they only need to store the headers beyond the latest checkpoint, CMIIW on this but that is how I remember it when checking out their codes. Might have to do with the way that they retrieve the headers, and thats why the zero data is necessary.
