Hi there. You sound a lot like my fiancé. He has been with me thru all the weight gain and not once has said a thing to me. I gained a lot of weight during my first pregnancy and then never lost it and then didn't care about my self enough to actually do something about it which I regret now. I am almost double the size I was when we first started dating. I believe your girlfriend can see how much she is gaining which is why all the comments about clothes shrinking. I think I have actually used that once or twice myself. :-/
When I first started gaining all the weight I used to find excuses why I was getting bigger/why my clothes were tight/outgrowing clothes. Thinking back now thinking back I think I wish I controlled myself more and looked after myself ALOT more then I did. Only your girlfriend can make the change, she needs to be committed to losing the weight and trying to eat healthier. Without will power and self control there is very little she can do. There's things you can do to help her like maybe pack some lunch yourself for her, go hiking on a trail sometime, or both do a hike and a picnic
its one way to help her. You can tell her your concerns but you have to put it lightly and I'm sure you don't want to say anything that will offend her. Maybe take her out on a date but not to a food place but to somewhere where is will be active, something she may enjoy. I am sorry I cant be much help.