It’s true for Vietnam, but like you I have doubts about it being true for Iraq or Afghanistan. According to the USSBS:
Which gives ~3.3 million tons of bombs dropped by the Allies in WW2. Presumably this doesn’t count the USSR, and of course doesn’t include the Axis. The Western Allies made more extensive use of airpower than other belligerents and were the only ones to conduct the tonnage extensive strategic bombing on a mass scale, but to be extremely generous and doubling the figure to include everyone else reaches ~6.6 million tons; the actual figure is doubtless much less. Vietnam saw
The four times figure referring to total bombs dropped by the US only during WW2. In Cambodia alone, warning, pdf file:
Which is more than the total tonnage dropped by the US in WW2 in both theatres. Its worth noting in explaining this that jet fighters in the 1960s carried larger bomb loads than 4-engined strategic bombers in WW2.