What happened?

One minute, I have this teeny tiny baby, smaller than a dolly, hooked up on wires, and not even fitting properly into preemie clothes/nappies. The next, fast forward 5 months and he is above average for his weight/height! He was 5 months yesterday, and weighs 17lb 3 oz now. The ped says we can start trying him on cereal, so we did.. He loved it!! Even opened his mouth up as the food headed towards him! How can this be? My tiny baby is not so tiny and growing up!