Don't punish the guy because he was being honest with you. Don't require follow-ups and ask questions and try and come up with clever ways to prove him wrong. It is his opinion. You can choose to consider or disregard it.
Weight gain happens. For women it usually happens around pregnancy or menopause. That was not the case for you, so there may be another reason - emotional or health-related one, and it's good to investigate. Personally I would be ok with a gentle "wake-up call", so I can start taking better care of myself before it has gotten too far and difficult to get in shape.
This subreddit only attracts a certain kind of bitter and, I'm noticing the same trend on ask woman too. Just chorincally online people who are displaying the real reason why they can't get humans in the real world to engage with them. It's ok to gain weight, find someone who's attracted to you or lose the weight.