I read somewhere that we've been using smaller JDAMs of late, certainly 8 500 pounders, with slightly offset impact points, would be more effective, in most situtations, than 2 2000 pounders. Since JDAM is really a guidance kit, it can be adapted to different size bombs.
I seem to recall that some of the bombs have the same sized casing; the amount of explosive is the variant.
Walt