Attached is a photo of a 1000 JSB 4,5 mm pellets that was air gauged. Marketed by Cometa as "Selected" pellets. I air gauged them and the measurements are expressed in centi meters of water colunm pressure. Dan Brown on US Benchrest Forum first came up with this idea.
Simply put, I measure the back pressure that is created when air flow over the pellet head by putting it in a taper hole.
From the photo it is clear that the diameters vary a bit. Taken that the 25,0 mm pressure is equal to 4,52 mm heads. Event though the heads may be the same size the weights still differ per size.