I don't know any technical stuff just my experience. I use sizing to make my stuff stiff especially my bookmarks before I sell them. When I tried starch it took for ever for it to dry. And I didn't think it was all that stiff. The sizing dries pretty quickly and I soak mine with the spray when I spray them. By morning they are really stiff. I use the sizing to stiffen a piece of fabric or to take out wrinkles. I iron it after spraying and it dries really well in just a couple of mins.
Maybe someone else will have a more technical answer.