A short wide foil will resist side force at lower speed. Imagine an airplane with a big fat stubby wing...like this:



It flies really well at low speeds - the fat wide wing generates a lot of lift without much air flowing over it. However, it has a lot of drag if you try to make it fly faster.

Now picture a very long narrow wing....like this:



Slope soarers / gliders are developed to be highly efficient but have to be at pretty good speeds to develop lift (compared to the lazy bee pictured above).

This translates into this: a short fat foil will provide the side force needed to control the boat off the start line - when it's moving very slowly in the water. A long narrow foil will stall and not generate much lift until it achieves a certain amount of flow over it's surface. This will make a long narrow foil very sensitive at low speeds and susceptible to stall.