Can you tell me what it means when on a fetus one of the ventricles is smaller than the other ventricle?

Could be normal. I am assuming that you are talking about ventricles in the brain. Ventricles of the heart are normally different sizes before birth. Regarding the brain ventricular size discreptancy-i am not very concerned as it may be a varient of normal. Many people are walking around with slight asymmetry of the ventricles-and they are normal. Probably would get a neurosonogram after birth to get better idea.