The idea of a "stubby" design is to:
- reduce the overall length and width of the board, to reduce swing weight and make the board nimbler in turns
- maximize the stability for a given length+width by spreading the volume on the larger area of a rectangular outline.(*)
- reduce the max paddling speed (drawback in faster/bigger waves), but thus lower the speed at which the planing starts (advantage in smaller/slower waves). In 4s period waves, a 7' will be in planing mode while a 9' will still be in displacement mode.
- have a more efficient planing shape, like a Simmons.
And people can size them in two very different ways:
- by increasing hugely the paddling stability by keeping roughly the overall dimensions they are used to. This is often what beginners do, or people having to go in nasty chop. But by going too big you get a cumbersome barge that will plow in chop.
- by reducing the dimensions while keeping the same stability. This is great for instance to try to reduce volume to get close to a 1:1 "guild ratio" (rider weight in kg / board volume in liters) for progression. Reducing dimensions reduces the weight and swing weight, and can become a killer weapon in small surf if you have the technique to generate speed (and it eases learning it). Plus it planes full in even slow waves. But as the waves get faster, it will plane too much and will tend to not hold the rail and skid all aver the place and spin out.
Basically I love my "Tomos/Stubbies" (Gong Fatal and One) for 7s period swells and less. But use more traditional shapes otherwise.
(*) Gong now provides the surface area of their boards and a volume+width distribution curve to help compare models. There are found as the last image in the set of pics of the boards.
E.g: Here is the one for the 105l Fatal 7'4" ("Stubby" shape):

Here is the one for the 105l Alley 7'8" (performance shape):