Canned Water In Plastic Shrinks. Why?

I washed out plastic milk gallon jugs starting about 2 years ago. Boiled Water, poured the water in the milk gallons. Put a piece of clear plastic between over the top, then screwed the top on. Some were placed in the house. Others outside on styraphone on the back porch. I covered them with Styrophome. I noticed several gallons have shrunk. Not leaking. But very tight as though the air had been sucked out of them. Why is this? Could it have been the heat? Cold? Those in the house are fine. Curious.