Interesting question and if you google there are several discussions from quilters to professional cleaners.
Sizing is also designed to create shape and dirt resistance.
A: Starch is used to stiffen fabrics such as shirt collars and cuffs, while sizing is used to add body, “crispness” and “hand” to garments. Starch is vegetable-based (it’s formulated from wheat, corn or rice), while sizing is a resinous solution that can be either vegetable- or plastic-based.
Because starch doesn’t satisfactorily adhere to thermoplastic fibers such as polyester, manufacturers often blend a plastic-based sizing agent and vegetable-based starch to impart stiffness to the synthetic fibers.