I had a friend with dwarfism. He was always so open to the innocent questions of children. He inspired me. There are different genetic reasons, and therefore different body types.
In general, little people have an average-size torso, but the long bones don't grow to full length. They have noticeably shorter arms and legs. The appearance of being overweight is in part due to the disproportion. But, they can enjoy too many calories like any of us. Weight gain can add to inherent spinal issues. He was careful of his diet, yet still looked overweight.
I'll go look for a link to help you understand the difficulties he faced.
Edit:  I found this:  "Some types of disproportionate short stature seem to have more problems with weight than others. For instance, individuals with achondroplasia seem to have a greater difficulty. Medical scientists think that this has something to do with the underlying metabolic disturbance in the achondroplasia. However, they also know that it is just as important for a person with achondroplasia to keep their weight under control as it is for anyone else tall or short. It simply means that individuals with achondroplasia have to work a little bit harder at keeping their weight under control."
http://www.lpaonline.org/assets/documents/Nutrition%20and%20the%20Little%20Person.pdf
Hope that helps!
