I'm in college for my A & P certification, so we've weighed a couple planes, and well, most general aviation planes don't weigh over 1300lbs. (even metal body cessena 152 is in the 1100lbs range.
Most kit planes are made of a composite fiberglass resin, and are extremely light.
It's just the fact that not all Gen. Av. planes are pushin 3, 4, even 500 hp! lol...
My dad actually was putting a Lancair kit plane together, and contemplated an LS1 for a powerplant. They're great on fuel, compared to other turbo-props and big bore piston engines, but the only draw back is that they are liquid cooled... otherwise, it's a pretty sweet setup for sure. I'm sure that is one siiiiiiiiiick plane!!!! (if it's even air-worthy.)