Whats the differance between GI and SGI irons?

Game improvement or Super Game improvement. Hmmm....tough.....

I've never quite understood; I mean Callaway Big Berthas are far more fat and ugly than X-series irons, and yet no more forgiving. G15s are SGI and yet a fairly conservative compared to some of the monster hybrid sets available.

JPX800s have finally got into the SGI category too, and they almost look like a regular cavity.