White String Bikini set - NWOT Size M but runs small

This is a white 2 piece string bikini that my daughter ordered online. She ordered a Medium but this fits more like a small/x-small. It’s solid white and ties together with strings. There are cute hearts on the sides of the bottoms and in between the cups. No stains, rips or tears. Comes in the bag it shipped to us in. Never worn but we don’t have the tags.