Do you not think that line of thought is a bit weird? As a person who has, at varying times, likely been described as "heavier", I'd be mortified to know you're looking at me thinking "good for her". It feels a wee bit patronising, even though I know you're not thinking it with any sort of ill intention. I'm genuinely not trying to be mean here or anything, it's just that these comments aren't as reassuring to many as you might think it is (I'm sure someone will come along and say "actually I really like this!" and that's good for them but they aren't everyone). Especially because when someone expresses embarrassment or nerves about being in the gym, people say "nobody's paying attention!" and comments like yours contradict that.