Not only do the definitions take up much more space, but the SDAT's also include software to detect and install the correct files for mulitple versions of VirusScan. I'm also guessing the SDAT has both the older and newer virus definition file formats.
The SDAT contains both V1 and V2 versions of the dat files as well as engine files for Windows (and some special engine files for certain Windows scanners). V1 dat files are end of life 31 March 2010, at which time the Superdat will probably drop by about 50% or more.
I know McAfee are doing stuff in the background to try and reduce the size of V2 dat files, but unfortunately I suspect that things aren't moving very fast with that attempt due to the number of new threats that they see each day.
I'd also guess that one issue preventing them from reducing the size too much is false positives. Reducing the size would probably mean less detail about a threat, and a greater likelihood of of a flase positive.