Eh, the best response is just living your life not giving a shit about their opinion and staying thin. Am a skinny pregnant woman who got a lot of "can't wait to see you get fat" comments. At 27 weeks I spend my time complaining about a lack of cute bump to take pictures of and telling all the fats how I can't find size 10au maternity jeans (but I can still fit in my normal jeans so I'll be ok for a little longer). Best revenge.