This one is a pun on the word "Megaton". "Mega", for those that don't know, is an SI Prefix meaning that the measurement is 10^6 (one million) times greater than the base measurement, and a metric ton is a thousand kilograms. One million times one thousand is one billion (1,000,000,000), so "Megaton" indcates a mass of 1,000,000,000 kilos. Which is about 2.2 billion pounds.