During World War II, the US developed some truly huge bombs. For example, the "Grand Slam" packs a 22,000lb blast (11 ton).
Today, however, it is unacceptable to use nukes. It is especially unacceptable to use them for taking out small targets (like underground bunkers). So the US has gone back to the drawing board and is developing new, terribly powerful, conventional bombs.
The first of these, the Massive Ordnance Penetrator (MOP), is a 30,000lb (15T) bomb. That is as powerful as Little Boy, but without any nukes. Which means it can be dropped on a battlefield without poisoning the landscape for centuries. Initial tests show that it can penetrate through 60' of concrete. My guess is that the goal here is to be able to take out Iran's underground nuclear development labs.