I'm curious as to why is my hard palate big like a knot?

Torus. You may have a benign overgrowth of bone known as a torus. This looks like a knot in the roof of your mouth. Tori can also occur on your lower jaw. It is a very slow overgrowth of bone. If a torus gets too large, it can be removed through a surgical procedure. See your dentist about this so a true diagnosis can be made.