Most plastic doesn't shrink down when you apply heat to it. What makes heat shrink tubing different is the process used to expand the plastic. This process is called cross-linking. Cross-linking exposes the polymers to radiation creating covalent bonds between atoms. This process is often referred to as giving the tube a "plastic memory". When the cross-linking process is over, the tube will remain it an expanded shape until heat is applied which it will then shrink back down to its original shape. Each type of tubing has a different shrink ratio that determines how small the tube diameter will shrink when heat is applied.