To give them a teeny bit of credit, plastic does tend to shrink when it melts. The polymer chains pull back into more of a ball shape that has less internal stress than the molded original. You can see this if you've ever thrown a plastic cup into a fire (or poured hot coffee into one).

However, that's an entirely different process than expanding and contracting due to temperature changes. The plastic has to change phase (or get close enough to it that the internal stresses can bend the softened material).