[Skunkworks] Of shrinking bottles

I have a question: The other day I put hot - as in boiling - water into a plastic bottle and it shrunk to almost half the size. This happens especially with Highlands juice bottles, av tested with Quencher and empty Coke 2ltr bottle but they are not affected much. I mean a 1ltr bottle literally shrunk to 500ml. Where did the rest of the bottle go?