Why do they shrink? Well The shiny chip bags are made with a piece of aluminium foil sandwiched between two plastic films –
The aluminium prevents air entering the packet and keeps the chips fresher for longer. The plastic in the chip packet is made up of long polymer molecules that have been stretched so they are almost straight.
When the plastic is heated in the oven, the molecules that make up the plastic are energized so they start to vibrate faster. The molecules end up wiggly, making the plastic film thicker and shorter. And thus your chip packet shrink.