NUKE, I see mathematics is not your strong point.
20,000 does not equal 1.5 million, I'm afraid.
20,000 is shreck all. More than 20,000 marched to keep fox hunting legal. Hell, there were that many protesting for an end to the bombing against Serbia during the Kosovo crisis.
So, with those facts established, it is hard to spin the line that there was even an iota of parity between the protests over Afghanistan, compared to Iraq.
In terms of a physics experiment, call the normal marches, a few thousand strong, 'background'. The march held in London on February 15th was a peak of huge amplitude. It was repeated across the world. It's definitely enough to write a paper on and get published. See what I'm trying to say?