Reflecting dramatic shifts in health care, Massachusetts General and Brigham and Women's hospitals are paying doctors more if they agree to accept new patients. The Harvard-affiliated hospitals are tying about 10 percent of doctors' salaries this year to the size of their practice and the complexity of their patients' illnesses. That means the hospitals' 360 employed primary-care physicians can increase their pay, now roughly $200,000 a year for those who work full time, if they see more patients than the average, or if they have many patients with multiple medical problems. Their pay can fall if they take care of fewer people.