How to tell someone they're getting fatter without being mean?

So long story short: when I first met my husband, he was pretty thin and fit. Now, a year into our marriage, he's gained a lot of pounds including a belly
which I don't really mind
but.. I really don't want him getting heavier since I'm just not attracted to that and since I'm such a health freak
how do I say it nicely?
How to tell someone they're getting fatter without being mean?
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