I don't know about it feeling empty and strangely hollow. I've sung in plenty of very large cathedral spaces, and even when they're packed with people they're still immensely resonant, sometimes to the detriment of the music - singers often prefer smaller spaces where the sound doesn't get lost in an acoustic bathtub. And as another commenter pointed out, there never would have been rows of pews in the Hagia Sophia like in a western church.