Metformin tablets if given in such large amounts can cause weight gain. Also, it tells that the insulin resistance has gone beyond expected limits and thus the more insulin in your body might not be able to control blood sugar, but might be leading to increase weight. Start walking regularly, with abdominal muscle strengthening exercises as exercises are shown to reduce weight as well as they help in decreasing insulins resistance. Also, consult an endocrinologist so that he uses multitude of medications so that your blood sugar is controlled without increasing weight.