Hi Will,
You have it right. If we take an 18ft long boat and stretch it to 20ft, we have added 2ft to the bow volume and that new bow volume is 2ft further forward with the 20ft hull. The boat now has more maximun pitchpole resisting torque, restoring moment. The maximum restoring moment has increased for two reasons, more bow volume and longer lever arm to that bow volume. Therefore maximum restoring moment increases as the square of the hull length. A the 20ft hull has (20/18)**2 = 1.23 or 23% more maximum restoring moment than an 18ft hull. This leads to higher top speeds and a more controllable platform for the 20ft boat.
More on these super light weight boats: If I take a 20ft boat at 375 pounds with a 325 pound crew and reduce the weight of the boat/platform 150 pounds, the lighter weight platform has lost (150/700) = 0.214 or a 21.4% loss in max restoring moment.To regain that lost restoring moment the hull length can be increased. How much? By the square root of the restoring moment loss or (1.214)**0.5 = 1.102 or 10.2%. This results in a 22ft light weight hull length with the same restoring moment as the heavier 20ft hull. I do this simplifed exercise to point out that when we make these lighter weight boats/platforms, "everything" doesn't get better. There are some performance/stability negatives.
Let's look at Platstation. What was the big modification to Playstation? It grew from a 100ft long boat to a 125ft long boat. This decision was done after a near "pitchpole" experience. The restoring moment was increased by (125/100)**2 = 1.56 or 56%. This was a major increase in restoring moment and it has led to much of the success of Playstation.
Bill