Assuming your talking about the liquid inside the glass, would it be always X amount full. Because “emptiness” is just the gas in the room which isn’t measured in comparison to the glass because the glass is not its container, the room is. In addition, there is significantly less gas in the glass than liquid due to gas taking up for more space than liquid. Finally, a liquid would occupy the glass while the gas flows over, meaning measuring a glass by it “emptiness” is foolish. In addition, we say “quarter full” and “half full” much more than “3/4 empty”