It's about wetted surface area....that's all. You are always displacing water weight equal to the weight of the crew and the boat combined minus any lift supplied by the sails (the spinnaker is the only sail that really provides any lift).

Suppose you are sailing the boat flat with both hulls equally 6" in the water... you have the thinnest part of both hulls displacing the water. If you move your weight to leeward and use more of one hull to displace the same amount of water, it runs deeper and more into the thicker/wider part of the hull. This means that the hull displaces more water with less penetration into the water than both hulls combined (because it's thicker in the water now). There's less gel coat exposed to the water because the hull profile is wider the higher you go. Less surface area in the water is less drag.

Putting your crew on the leeward trap will increase mast compression and sail tuning slightly as a result. However, the big picture is the lateral center of gravity of the crew weight and the boat weight. The more weight to the leeward side, the more weight the leeward hull carries (and displaces water) and the less the windward hull deals with. We all know that putting your crew on the leeward trap (especially lake sailing) is like sitting in a tree with your kahunnas exposed above a pinatta party!