I understand that generally breastfeeding will help to accelerate the process of losing weight but at the same time i find it irony cos there a quite a bunch of my friends who is bf but they actually gain weight than losing them. Why is this so?

Yes I read about this and this is actually quite common too. In general breastfeeding will help us to shed weight. But that's the problem. Many women think that by breastfeeding they can lose weight so after delivery they just happily eat what they want thinking that it is ok since they are breastfeeding and should be allowed to eat heartily. In the end they gain weight instead. I have seen it happened to many of my friends before.