I don't know much about larger planes, but a good flying .40-.45 sized sport model with 600-650 sq inches of wing shouldn't weigh much more than 5 pounds. A 500 sq inch model more like 4.5 pounds tops...4lbs is actually very doable.
Every oz above these figures will just make the models less capable, more treacherous and less fun.
"It" has less to do with your thumbs and more to do with the plane and the way it is set up.