Two Pennsylvania physicians offer $100 discounts to patients who quit smoking during pregnancy, citing evidence that babies born to mothers who smoke are usually smaller than normal. The doctors, from Parkside--a suburb of Philadelphia--report some success in their effort. Of about 100 patients who were smoking at the time, 75 took them up on their discount offer and pledged to stop smoking and 40 of those 75 were successful in quitting. The doctors, Irving A. Mann and Robert W. Haggerty, said they would continue the program and hope the percentages will improve over the years. Dr. Mann said he thought they got larger and healthier babies as a result.