Just for argument's sakes, i looked up HR/game numbers for AT&T and Fenway Park:
Park factor has AT&T leading Fenway by a 0.936 to 0.907 value, for 0.029 difference, which in Park Factor, is somewhat of a noticeable difference.
However, HR/Game has Fenway beating AT&T park by a 2.18 to 1.48 margin, which is an enormous difference.
In fact, the only stadiums which have a lower HR/Game ratio than the Giants this year are Turner Field, Busch stadium and Dodger stadium.