Is it meant as flirting if someone says "no big deal if you're gaining weight"?

I would think it's quite possible. Like it or not, weight gain often happens in a very comfortable relationship, so much so, that I tend to think there's a bit of an association with it. Someone saying that to you could mean that they could imagine being in a comfortable relationship with you some day. At the very least, they are commenting on your body which is always a bit flirtatious, if not done condescendingly.