The largest air-deliverable conventional weapon in the world is a Fuel-Air Explosive. The BLU-82 was developed by the USAF during the Vietnam War. For what, exactly, only God and the propellerheads in some basement in the Pentagon know.
I can see it now. This thing is built by the same guys that made their own black powder bombs in high school, except now they have PhD's in engineering and a black budget bigger than the GDP of Egypt. They're sitting around the basement, looking over bomb damage assessments and autopsying the frogs they've "prosecuted" with an experimental backpack microwave gun:
Propellerhead 1: Hey Tom, you did some work with FAEs, right?
Propellerhead 2: (looks up from frog dissection tray) Yeah, back in '68.
Propellerhead 1: What's the biggest one you ever made?
Propellerhead 2: We had a 2000 lb unit that we would drop of off an F-4...what are you thinking?
Propellerhead 1: Well, I was thinking - A C-130 cargo plane is a hell of a lot bigger than an F-4...
Propellerhead 2: I've got you! The density is pretty low on those units, the real upper limit is volume. We could make one the size of a boxcar!
Propellerhead 1: Yeah! Roll it right up the back ramp. At that size the payload would be about...
Propellerhead 2: (madly working slide rule) 15,000 pounds!
Propellerhead 1: Holy Shit!
They used one of these in the Gulf War. It was a distraction tactic to misdirect the Iraqi's from the armored Hail Mary that would constitute the ground war. For 3 days, psyops dropped leaflets on the Iraqi positions that roughly read (in Arabic):
BIG BLU is coming! On Thursday, the largest conventional airdropped bomb in the world, the BLU-82, will be dropped on your position.
I guess the intent was to hurl the golden apple of discord amongst the ranks of the Republican Guard, and send a misleading message to the Iraqi central command. When the bomb was dropped, right on schedule, a British SAS recon team broke radio silence to report:
"Command, Command! I think the blokes just nuked Kuwait!"
I orignially misstated that the leaflets were in Farsi, which apparently isn't the language spoken in Iraq. Iraq = Arabic Dialect. Iran = Farsi.