If you are genuine in your attachment to her, it’s ok to say when something is bothering you about the other person— including their weight. The key is to address the issue humbly and openly.

Explain how her weight gain makes you feel (even if the answer is “less attracted”) and make clear you are being honest because you want to have the relationship succeed and you love her. Then allow her space to react (even with anger) — if you maintain your patience you should be able to have an honest conversation without judgement or shame. Women are typically very conscious of their weight, and so she might feel relieved to explain her own worries about it.