The counterweights have nothing to do with the pistons and rods.

I know.

Fully counterweighted means that each throw has two counterweights that are half the size. With single counterweights the bending forces load up the one main bearing, With two counterweights/throw the load is shared evenly between two main bearings & there is a lot less twisting going on & everything can be smaller & lighter.

Of course, all those forces aren't quite lined up

Makes them jump up & down somewhat.

The very common Mitsubishi 4-banger here is the 2.6litre 4G54. They have a balance shaft that counter this very well. Porche bought the rights to Mitsubishi's balance shaft layout so they could use it in their big 3.0 litre 944 4 cylinder.