Goober, would it be possible to use
something like QuickPAR (see MultiPar, below) to minimize the download sizes?
QuickPAR uses par files sort of like RAID stripes -- I don't know how, but you can change the original file anywhere and do anything to it -- the only caveat is the par file needs to be generated so as to be large enough to cover the changed data.
I don't know how you would make it work, but QuickPAR would basically allow you to only download the amount of file size that was changed between the original and modified versions.
You'd have to figure out a way (if possible!) to make QuickPAR either patch the original files to new versions, AND/OR create new files from the old ones.
e.g.:
Updating to MVPs 2014 from MVPs 2012 while keeping 2012, you'd only need to (on the dev side) generate a par file for the 2014 MVPs, download that par file (on the user end), then either have QuickPAR copy the new files to the 2014 directory (alternatively, just use a copy command to copy 2012 to the new 2014 directory, then QuickPAR the 2014 directory up to date).
I think this could be automated with the Installer. Not sure about the amount of time needed to be invested in a project such as this, but I imagine the bandwidth savings would be immense. Especially for updates that are small (1-100MB in size) but would normally require re-downloading the entire mod (1-5GB).
EDIT:
parchive par2cmdline (DOS, Win32, OSX, Linux, source etc etc).
EDIT2:
Plans are also to introduce PAR3 (support for directory structures among other things.)
EDIT3: I'm behind the power curve. QuickPAR has been superseded by
MultPar, which has support for PAR3