My guess is that the deciding factor is the throat.

Yuo can have tiny ears if it would be better, but you need a throat of the corrrect size for breathing and eating.

If the longer wavelengths are useful because they aren't impeded by obstacles like masses of forest leaves, then the same properties probably mean ear-flaps aren't really effective to shape/concentrate them either. (Although they could be used as a filter to block out higher frequencies.)

I think the large ears are for cooling the big bodies.