Are you going to give us any hints? Which update?
However, I'd expect something like that if the update doesn't install any files. Or even if it installs files that replace an existing file (as an update might do).
A firmware update may be large, but what it "installs" isn't a file and doesn't take up any disk space. Similarly with an update to the "Recovery HD" partition (which, at 650MB, is just about the right size).
An updater that replaces one 730MB file with another 730MB file would likewise not need any (additional) disk space, especially if in the remaining 5MB it verifies that the old 730MB file is present and refuses to update in its absence.
It's conceivable for an update package to contain no files at all (in the ordinary sense), if its payload consists entirely of install scripts.
The 73k is probably the size of the package receipt.
