At lower engine speeds, keeping the velocity of the air high as it discharges around the intake valve while at the lower lifts(during the overlap period when the piston is moving in the "wrong direction" as far as the intake port is concerned) is easier to accomplish when the valves are smaller rather than bigger.
Like with most things horsepower related, there is going to be a crossover or tipping point for each combination.
With regards to intake valve discharge coefficient(i think especially in a restricted exhaust application), too big of a valve allows for more reversion, and too small of a valve limits peak power output.