Toddler eating habits.

My 18 month old eats like a bird. She always eating really good at breakfast, but she barely touches her lunch and dinner. She does have snacks during the day but not so much that having them ruin her meals. She's not losing weight but has only gained 1.2 pounds since her 15 month check up, the doctor says it's because she's so active (my child doesn't know how to sit down!) Does anyone else have this problem? I'm just worried she's not eating enough.