If someone you are sitting next to makes a remark to you, you can be honest in saying you have a medical condition that causes weight problems. I am familiar with PCOS, the fact that it causes weight gain, excess facial hair etc., and it isn't exactly something that is in your control. Not all weight gain is about 'go do some exercise', unfortunately it isn't as simple as that, and a lot of people don't seem to realise. All you can do is be the better person.