Size,weight and power limit the designs. Look at the change of airplanes back when they did away with the displacement rule. It wasn't uncommon for a .60 size plane to weigh 10.5-11 lbs back in the 80's. We learned to reduce weight, those airplanes could easily weight under 8 lbs. Today given the size it's very uncommon to see a sub 10lb airplane. Back in the 80's we thought there was nowhere for the designs to go yet here we are today. Turn around style and Engines have made the biggest impact on our sport.