breaking up with her due to her weight gain is not being shallow. Relationships are built on attraction, otherwise they would just be friendships. Your gf is not the same person that attracted you like she did when you two started dating. Unless she loses weight soon, the problem will only get worse. This is also a sign of things to come. She's already let her weight go b/c she became complacent with you. What else might she change in the future?