Iron with a low-medium temp iron while still slightly damp. Silk fiber is a protein, like your hair, and it does not itself shrink. The way the individual fibers are twisted together when weaving is what causes silk to shrink. Highly twisted yarns and loose weaves cause shrinking when water releases twisting energy in the fibers. It's a bit like twisting a rubber band then reducing the length, seeing it bunch up.
Silk bunches up the same way.
Ready to wear silk garments shrink because manufacturers don't go to the trouble of washing the fabric first. Silk Road, inc.