My 19 month old is very energetic and bubbly. She eats well, and isn't really fussy: she will eat protein, fat, carbs, veg, fruit in good proportion. She prefers savoury. She's short for her age but both parents are naturally short and slim. No allergies, bar slight dry skin from some bath products.
Since she started walking/running/climbing she has lost all her baby chub and become really slim and wiry! I worry people will think she's ill or underfed. 
She eats 3 good square meals a day plus snacks, and is gaining along her line in the red book.
I'm being PFB, aren't I? Some kids just look wiry? I think it's because she was a chubby baby a few months ago, and also i never see other toddlers in the bath etc so cannot compare really (especially in winter clothes).
To put in in context I'm 5"1 and weigh 7stone 10lb, perfectly healthy. DH is 5"7 (he says 8 LOL) and slim-with-a-beer-belly even though he's a lazy so and so who could do with exercise.
She goes to nursery daily and no one has raised any concerns, even when I ask. They say she eats like a horse and runs it off.