[norcalvet]

I seem to recall that some of the bombs have the same sized casing; the amount of explosive is the variant

Nope. Different size bombs have different quantities of explosive fillers. Most laser-guided bombs and JDAMs consist of guidance units and tail sections mounted on Mk-80 series "dumb" bombs, making them "smart" in the process. Net Explosive Weight (the actual explosive material in the bomb) is roughly 45% of the gross weight of these bombs.

Bomb              weight (depends 
                        on fin assembly)
Mk-81             260-305 lbs.
Mk-82             531-565 lbs.
Mk-83             985-1105 lbs.
Mk-84             1970 lbs.

[norcalvet]

BTW, these weights are slightly heavier for JDAMs and LGB's that have additional guidance and control sections.