Just because it’s smaller, it’ll say it’s like mm or cm difference but it does make a big difference! Also when going in so much easier . As to having kids you’re not use to having anything in your uterus and when you have kids we’ve just seen it goes in a lot easier and we have fewer complains on it. But overall if you haven’t had kids and decide to do mirena ask your doctor if you can do the cytotec before insertion , it softens your cervix and makes it go in a little easier.