My overflow plumbing was always 1",it was my return plumbing that I changed over from 3/4".I don think there was much difference in the amount of air that went thru the overflow,but the amount of water was about 40% more.It was really remarkable,In fact,my tank overflowed at first until i added another
u-tube to keep up.I really need to get another tank with built in overflow,these HOB overflows are a P.I.T.A. IMO