It is unlikely that she would lose weight - had it been possible, she would have long done it. Assume that she will stay the same for some time ane then might gain a bit. Be realistic - will it be fun to stay with her in the long haul.

It would certainly be cruel to marry her but not be excited about het. She sounds like a lovely person who deserves an enthusiastic lover. Can you be that lover? What I am saying is that leaving her and living with her without enthusiasm would both hurt her. Her reality is that she is in love with a man who is not in love with her (adores her but is not in love with her). It is a mild case of unrequited love (in the mildest form). Whatever you choose has the potential of hurting her, and it is hard to determine which hurt would be bigger. So think of your own interest first.