As John said everything in designing a boat is a trade off. Sounds like you traded stability for lower resistance. So your not really eating the cake, your trading it for something. In this case, lower resistance for stability. If you modify the design to have the same stabily (or lower the 20' version) your probably going to see what you expected. If you widen the boat or increase the stability some other way you will increase the wetted surface of the boat and therefore increase the resistance.

I created a model in my software I call "Basic Boat". I have taken that model and stretched it and shortened it without changing any other factors (yes other things have to change when software re-sizes the boat). It's very interesting to do that and see what effect just changing length of width does to the resistance and stability numbers. Unless you race, longer it not always an advantage. I am not into pain and suffering .... I mean racing. :), but it looks to me that you could take this info and fine tune a design to minimize the resistance in your speed range.

I designed a boat that way. My target speeds were the 2.5 to 4 mph that most recreational paddlers stay in. I tried a lot of versions of the boat, different lengths and widths and found the sweet spot of the style of boat I had in mind. The 15' model was the best as in had the lowest resistance in the speed range and stability numbers I wanted.