In other words, you relate time to how much time you have already lived. That day takes a relatively long time compared to the rest of the baby's life. When you are 5 years old, you have lived about 1825 days. That seems like a long time. When you are 10 years old, you have lived about 3650 days.
Days are much smaller, so time goes by faster than before.
When you are 50 years old, you have lived about 18,250 days. That is not much, so it goes by fairly fast. Of course, days go by much faster. There is a joke that when you are young, time goes by fast when you are having fun. But when you are old, time goes by fast whether or not you're having fun.