My eyes change size throughout the day. What is the cause of all this? Also, is there a way to make my eyes stable?

Hello, I notice that my eyes change size throughout the day. When I wake up, they are medium, but when I return from school, they become large. Also, after I read a book, my eyes tend to look smaller. What is the cause of all this? Also, is there a way to make my eyes stable? As in big when I return from school? Thanks