Body Question, Help appreciated!?

I can't figure out my body shape and if it's normal like this, I have a relatively flat stomach, I weigh 112 pounds, 5'1 in height, weight height chart says I have a normal weight-height ratio. Problem is, I can't figure why my arms and legs are bigger than normal. They are huge but I know I am not fat. What is it that made them that way? Fat? Muscles? Should I exercise or cut back on eating junk foods, carbs? My stomach seems to be fine, it's not that flat but it's not huge either. But my arms and legs just doesn't seem proportionate to it. Help? I am going to start running/walking from 530AM-700AM from now on, because I am to attend a wedding and I want to look good. Will this help?