According to Wiki, the RAF dropped nearly a million tons of bombs during WW2. The americans "contributing" a further 600kT.
Another source puts the total WW2 amount, dropped everywhere. at well over 3 million tons.
But it doesn't end there! If the researchers wanted to investigate more instances they could look at Vietnam. During operation Rolling Thunder the americans dropped 864,000 tons on the north.
Amounts so huge, that I simply can't process them.