I wonder why my baby isn't gaining much weight despite the fact that he drinks a lot of milk, and eats Cerelac twice a day. I expect him to be plump, but he's getting taller instead.

See, you need not worry because your baby is showing signs of growing up in gaining height. As long as your baby is active and eating well, you need to take tension. Some babies are plump and some are not. It all depends on the body type of the baby. Also because he is gaining height, is the reason he is not being plump.