Well, I think there's a question of boundary.
According to the 2012 Census data, It is bigger:
- Philadelphia "metro statistical area": 6.0 million
- Boston "metro statistical area": 4.8 million
http://en.m.wikipedia.org/wiki/List_of_Metropolitan_Statistical_Areas
That said, I'm not quite sure how they're defining the Boston metro area for the purposes of the TV market. It seems like Boston's statistical area doesn't include Worcester, Hartford, CT, or Portland, ME (but does include Rhode Island?).
Anyway, I think the Philadelphia "area" boundaries are complicated by the presence of New Jersey and Delaware suburbs, whose residents may not have any real affiliation with Philadelphia for sports watching purposes.