Most Helpful Girl
There is no way to ensure that your significant other won't get fat. They are a free person and free to make their own decisions. It's not like an animal where you can purchase them only diet food and force to take regular walks lol. If it's that important to you, I would encourage you to only get serious with women who appear to be health conscious. Some people make eating healthy and getting regular exercise a big deal... marry one of those people. Also, don't assume that just because a woman is 'skinny' that she will remain that way. I was never the 'skinny' girl in school but now, even after 2 children, I am by far in better shape that most all the women I graduated with. Why? Because I have always had to be health conscious and exercise just to stay 'thick' and not fat. Skinny girls get older, have children, and then have no clue what it's like to have to diet or workout.
Most Helpful Guy
Tell her she's gaining weight. Or remind her of the plethora of health problems that accompany being overweight/obese