I'm pretty sure she spills most of it I just went off of what I refill daily... but I'm not worried as she has a very healthy diet and has got a clean bill of health from the vet recently!16 oz is kind of a lot for a little 2 lb bunny and doesn't seem normal to me. I would be worried there might be a possible health issue there. Does she maintain a good weight still, or any other signs of a possible health issue like creamy/gritty/gel like thick urine, wet bum, drooling, dropping food from her mouth, not eating her cecals, etc?
Do all of your other rabbits drink a lot as well, or just her? Generally water consumption is about 1 oz per lb of body weight, but that can vary a little depending on diet, ,environmental temp, and the individual rabbit. I would say 4 oz/lb max, but even that is getting to the point of being excessive. Over 8 oz/lb is a lot for a rabbit to be drinking.