A boisterous political battle over a proposed expansion by the largest and most expensive hospital system in Massachusetts is spotlighting questions about whether similar expansions by big health systems around the country drive up health care costs. Mass General Brigham, which owns 11 hospitals in the state, has proposed a $2.3 billion expansion including a new 482-bed tower at its flagship Massachusetts General Hospital in Boston and a 78-bed addition to Brigham and Women’s Faulkner Hospital. The most controversial element, however, is a plan to build three comprehensive ambulatory care cent…