My wife is putting on weight, how would you broach the subject?

I love my wife and I am okay with her being overweight. She never was and never will be thin. But in the past several months she has been putting on more weight. She exercises, though inconsistently. I think the major problem is she likes to eat. When we eat and I am finished, she often goes for another helping. I won't love her any less if she weighs more, but I wouldn't mind it if she weighed less. How would you address the issue if you were in my position?
My wife is putting on weight, how would you broach the subject?
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