How to convince my girlfriend were unfit?

Me and my girlfriend have been living together for around 4 years, when we moved in together we were fit and skinny. Slowly over time we have both gotten lazier and we have both put on quite a lot of weight, I've put on more. Everytime i bring it up or complain about how tight my new trousers already are, she just says its fine happy couples always gain a bit. But it's not just a bit anymore i wanna go back to how we were
How to convince my girlfriend were unfit?
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