Well, if you time it right a hand grenade will expand in mid air!!!
A shoulder fired net, like they use to catch live critters, from airplanes, will expand in mid air.
Buckshot expand's to a nice pattern in mid air.
chemical agents expand in mid air
I know this is probably what you are looking for, But if you throw a solid object and it expands in mid air its going to create mass and loose weight and it's velocity, is going to go strait in the dirt, very quick, so that rules out most impact type weapons, you would almost have to have a secondary charge to deploye the expansion, after you have thrown your device, weather it be chemical or a fragmentation device, you might try looking up non-leathal weapons there might be somthing worth looking at there. Good luck. hope that helps.