Look at the bottom V A is constant speed. So, the only thing we have to play with is A 0. If we want to reduce thrust loading coefficient, the only option we have is increase A 0. What is A 0? We have written here is proportional to or diameter square; that means, the bigger the propeller, the higher will be your efficiency. Do you get? So, that means this is conclusion one and the conclusion two is for higher efficiency increase propeller dia.