Regarding making Righting moment as the determining factor in what regards stiffness, independently of the boat weight, that would mean that a considerable bigger and heavier boat would be always more stiff than a lighter one and that is ridiculous.
Keep in mind that the Catalina 400 was marketed as a racer cruiser when it came out in 1995. It has a radically different hull than the 387 and quite a bit different hull than the 42.
It represented a full bodied, wide beam hull that focused the hull mass deep in the center of the hull.
The First 40 as I pointed out earlier focuses on low drag at low angles of heel rather than stiffness. We see this in the side view of the hull as the depth of the hull is relatively flat. Such a design combined with a narrower waterline beam attempts to minimize the wetted surface. This would become clear if you had ever compared the front views.
On the other hand, the Catalina hull side view concentrates the depth of the hull near the midship. This lowers the CG. Clearly the combination of lower CG and large water line area results in a very stiff boat at low angles of heel.
The First hull sides have deadrise that creates an increasinly larger RM once the boat starts to heel beyond perhaps 15 degrees.
On the other hand, the Catalina 400 hull sides are more rounded resulting in a more progressive and easier to manage increase in RM.
So we see, it is not really the weight of the Catalina 400 that results in its stiffness at low heel angles, but more so its hull design.
The Catalina 400 is nothing like the hull you are apparently expecting. I encourage you to see these two boats out of the water to get a better understanding.