If the body has skinny legs/arms/other body parts, its more than likely Blendshapes to prevent clipping while you have clothes on. If you need to see the body for something, click on the Body until it highlights orange. In the Inspector tab there will be an option called 'Blendshapes' with an arrow next to it. All of the slider options are basically animation build into the avatar. Change whatever slider causes thin body parts and BE SURE TO PUT THEM BACK TO ORIGINAL SLIDER POSITION BEFORE UPLOADING.