Both of my peditricians have said that toddlers know how much food they need to get through the day. They are so busy with other things. My daughter is 3 (almost 3 1/2) and still only weighs about 25-26 lbs. She has always been small. In her 1st year the main concern was that she not drop off her growth curve which she did not. Now that she is 3 her percentile jumped from below the 5th to the 10th because they just don't grow so much after the first 2 years. Ask your pediatrician.
I do try to make the food a little more creative looking. Kind of make things look like faces, boats, etc. Breakfast is just not her meal. So, to make sure she gets some calories I mix her milk with Carnation Instant Breakfast (or the generic brand). Sometimes I mix in some fruit. Now my daughter does eat a variety of fruits, veggies and meats. However, she will go a day or two eating like a bird and then she seems to refuel with larger meals.
Good luck. I know it is frustrating. You wonder how they can have so much energy on such a small amt of food. As adults we seem to think we need to eat 3-5 meals a day. And, like my pedi. said, we could learn a lot about eating from toddlers.