Doll makers use crumpled aluminium foil, epoxy sculpt, Ultralight clay and other materials as needed to add bulk to their figure. These materials are applied onto the wire armature. After bulking out, the armature is covered with the top layer of polymer clay ready to sculpt (the top layer is approximately 1/4 inch thick). This technique is sometimes called an under-sculpt.
Once your sculpt is baked, additional bulk can be applied prior to covering with a costume using soft materials, such as cotton or poly batting. The soft material is placed where needed and held there using a gauze wrap.
For more information check our topic 'Under-sculpt'.