Think of it like a rubber band. When the glass is empty the rubber band is loose. If you pluck the rubber band it will give you a low pitched “boioioing”

As you tighten the tension on the rubber band it gets higher pitch and less boioioioing and more “boing boing bing bing bin bin” tighter rubber band- higher pitch- less vibration.

Same thing but with the cup being the rubber band. As the water fills there is less cup to vibrate.