How big is the GBU-43 bomb that the U.S. dropped today on an ISIS tunnel complex in Afganistan?

It's more than 10 times bigger than the next biggest bomb in the U.S. conventional arsenal, but not big at all compared to a nuclear weapon.

Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists, said the explosive power of even the smallest U.S. nuke, the B-61 bomb, is "an order of magnitude" larger than the GBU-43.

"The smallest nuclear weapon in the U.S. arsenal has an explosive yield of 0.3 kilotons of TNT, meaning 300 tons. This bomb, at 21,000 pounds, is only 10 tons. It doesn't come close," said Kristensen. "Even the biggest conventional bomb we can load onto a plane is miniscule."

The B-61 bomb, only deployed in Europe, is a tactical weapon that can be used to destroy city centers or large-scale troop concentrations.

The GBU-43, also know as a MOAB (Massive Ordnance Air Blast) is also about half the size of the smallest U.S. nuke ever built, the Davy Crockett artillery shell, which was retired in the 1960's.

Ironically, Thursday's bombing occurs during a defense community debate on whether to build smaller nukes. "We have people arguing for new mini nukes," said Kristensen. "Here you have a case where the U.S. felt all it needed was a conventional whopper."