Why do Asians have slanted and/or small eyes?

Does it have something to do with heredity?I have bigger eyes than average asians but I do see asians with slanted or small eyes.Why is this?I also have double eyelids in my eye.My mom gets proud of my eyes.Why,is it consider good in asian cultures to have big eyes and double eyelids?Do asians have small and slanted because of the environment?