AFAIK, DevIL.dll is the official binary for that rather than one that's custom-built (although I could be wrong).
The binary *.dlls themselves are in the Git repository, in already-UPX'd form, and just get copied around afterward. An unpacked DevIL.dll is 2 megs in size.
Anyway, my decision to UPX the binaries is based mostly on tradition and habit. After really considering it and looking at some of the adverse affects (I don't consider the anti-virus argument an important reason; the increased memory and loading complexity arguments are important, though), I don't think I'll do that anymore. It was never actually based on the bandwidth argument or disk space, really.