What does it mean that my gallbladder is physiologically distended?

Fasting? It may just mean that it's full of bile. If you refrain from eating for the Ultrasound your gallbladder will fill with bile and become physiologically distended. That's normal. When you eat the GB contracts and pushes bile out into your intestines and therefore would physiologically empty. . .
Depends. Usually when the GB is distended it is usually seen when a person is not eating. Generally a is a incidental finding and does not need anything unless a patient is symptomatic. It also depends on what modality was used to define the size of the gallbladder, whether it was an ultrasound or CT scan. Unless you are having symptoms of discomfort in the right upper quadrant, I would not worry.