This is just an assumption on my part but I think RC engines have gained a lot of weight simply to make them strong enough to survive crashes. Similar sized CL engines are very much lighter because all up weight of the model is important. The Brodak 40 mentioned in the OP was designed as a CL stunt engine and its all up weight with muffler (of my engine) is 8.2 ounces. I have an ABC rear exhaust CL stunt 60 that weighs about 13.5 ounces with muffler. There's also a lightweight version of that engine too and a RE 81 that weighs 13 ounces without muffler. Jett has a rear exhaust 60 that weighs 13 ounces (without muffler) so it can be done.