We left these outside for a week and they shrank so much they became unwearable. We bought them a couple sizes too big for our daughter (her feet grow fast) to wear in the back yard. We left them outside and now they don't even fit her younger brother. They didn't shrink uniformly all over, they mostly caved in at the top.

Perhaps the sun has some effect on the plastic. I don't know, but it reminded me a little of Shrinky Dinks (yes I'm old).