Partly size, but it is more a money issue. It is extremely expensive to educate and train a pilot and US doesn't mind spending a little more. Either in extra training needed or just by accepting more "dropouts" during the actual training.
They also have some stupid monority rules that tells them they have to have such and such a percentage of each minority group. This in some cases make them select students that in my opinion would have been better of doing something else.