We have both put on weight over the years. Mine has fluctuated (gone from a size 16, down to a 12, up to a 16, down to a 10 and back up to 16) whereas he's just gone up and up and up. He wears XXXL clothes.
It's not something I notice that often nor does it cross my mind all the time, but every so often his largeness catches my eye and I think ugh. He really needs to do something about it. Not to make him more physically attractive to me, but for his own health.
As PP have said its not the actual weight gain that is a turn off, its the attitude that comes with it. He's just lazy about his health and that is a turn off TBH.
I try with my weight and for me it is mainly food. DH is a feeder. He says to leave what I don't want, but once its on my plate that is much easier said than done. I do exercise though (mainly running with the odd phases of weights/bootcamp/gym)
I just wish he would try to do something about it. Every so often he has a go at something, but it never lasts long. I also don't feel I can force him.
Am I going to leave him for it? Hell no. He has many other wonderful qualities about him. However, if something happened and I found myself single and was looking. A deal breaker would be someone who is fit and healthy and enjoys some form of exercise. DH never really did throughout the time I've known him.