You could make them lighter. Use carbon and composites for the chassis. Thin down the arms and shock towers. Use something besides steel for the gears. Make the diffs smaller.
The problem in the end is durability. People that drive 1/8th scale cars don't want to break an arm every time they make a bad landing or tag another car. Also, the tracks designed for 1/8th are just plain more brutal on the cars than tracks designed for 1/10th. Bigger jumps, rougher surfaces, longer tracks, and higher speeds all contribute to the need for stronger cars.