Jordyn

My son went from being in the 75th percentile at 4 months to the 20th at a year and the 10th at 18 months. He is a VERY picky eater but at that point he would eat things that were spicy and savoury (funny he won't now though) and his paed was very concerned about his weight and the possibility of cystic fibrosis. However this was extremely cautionary and only because dh is adopted and we have NO information on his birth parents. What she did tell us at the time (as did friends with a child with cf) that children with metabolic disorders usually have some odd eating habits, have obvious signs of being unhealthy and usually have some type of stomache issues such as intense gas pains, diarrhea, constipation, mucousy stools, frequent vomitting etc. Even children with failure to thrive concerns are not just underweight/showing slow growth signs but have pale colouring and aren't meeting developmental milestones.
Hopefully this isn't a concern for your child...doesn't sound that way since eating doesn't seem to be an issue. My mom says just wait, in 15 years when the coach potato-ness sets in we'll be wishing they were skinny and active again!!!