Why do cats eyes get smaller when there in the sun but big when its dark?

When it's cold, the erector pili muscles that make hair/fur stand up are active. (They're also the reason why your body hair stands when it's cold, and you get goosebumps.) The fur traps air in the spaces between individual hairs, which then act as an insulator to prevent heat loss from the body. On hot days, the cat wants to cool down so the fur lies flat to prevent any insulating air from being trapped in the fur.