Good question! DF are pretty open about things like this, so we would probably say something before it got too far. We've had conversations before that we need to kick start our healthy eating as 'we've' put on a bit of weight / have been slack lately. Usually we reach that point at the same time as we eat pretty much the same diet.
I really struggled last year when I was diagnosed with Hashimotos as I was putting weight on despite exercising and eating right. It was the first time I had ever struggled with my weight, but DF was really supportive.
I think it comes down to what you're relationship is like, like a Pp said. This convo could be a huge fight starter in some relationships and others it wouldn't even be an issue. I think the wife who expects her husband to be under a certain weight though it taking it a bit far.