Pidgeot: That wouldn't help. Installers use compression, so even a tiny change can (and often will) change the entire file. Actually, the downloader already does that to an extent; it downloads chunks of XMB (50, I think?), and if some of the chunks are already present and matching the current file, they won't be redownloaded.
niky45: ... well, they would of course have to change the download method, from a monolithic file to a file tree (say, a folder). then the compression could be used in the connection and not on the file (... rsync for example does this). that way, we could have both benefits of compression AND diffs.
... I think that is what steam does.... and personally, I like the way their client handles downloads.
Yes, and that's what Galaxy will be doing, too, and it is definitely something I'm looking forward to, myself.
Outside of Galaxy, however, there's not much else that can be done.