Megapixels describe the resolution in a generated multipoint image.
This is the width multiplied by the height so 2000 x 2000 = 4MP.
We calculate MP using the resolution in meters and the radius. If you request a 10m resolution calculation at 10km radius this is (10,000 * 2) / 10 = 2000 points. 2000 x 2000 = 4MP.