I take 300 in the morning and have never had weight gain but I do run and watch what I eat - i limit my portions - everyone is different
I take Lamictal every night - 50 mg. dose and have for several years. I didn't see any increase in weight or appetite. However, I do take mirtazapine(Remeron), and have gained over 10 lbs. on it (taking for 11 mos. now). Not surprising as the appetite increase/weight gain can be a drawback to Remeron.
Have you asked your Dr. - perhaps it's the combination? Maybe ask the pharmacist?
Hope you find the answer...it's frustrating, I know...