Size meant nothing to Lee. The man was built of stone. He overdosed on a drug because he had no body fat to absorb any of it! Lee was said to be one of the strongest men P4P in the world. I dont think GSP having 20 lbs on him would mean anything to be honest. And who said Lee didn't grapple? Have you seen any of his sparring Vids? His akido was amazing.
You seem to miss the fact that JKD is one of the precursors to MMA...
Bruce Lee was an absolute beast and personal hero, in fact I am wearing a Bruce Lee Shirt right now. But, GSP had a lot more than 20 pounds on him. GSP probably walks at like 185-190 and Lee weighed 135 to 150 at his heaviest.