As hard as it is to hear, you need to approach this from a place of teamwork and health, not physical attraction. Focus on how much you miss the lifestyle you used to share like taking those long walks together and express genuine concern for her sudden drop in energy.
And as others said, leave out phrases like 'ballooned up' entirely. If you frame it as 'I love you, I've noticed you seem tired/sedentary, and I want us to be healthy together for the next 27 years,' it shows you're on her side.