Because the moon travels along an elliptical path, sometimes it gets closer to Earth, which makes it appear larger in the night sky. This is known as a Supermoon and tonight it will be the biggest and brightest of the year, because it is full closest to the point of perigee. Perigee makes a full moon appear up to 7% larger and 15% brighter than a typical full moon.
Be ready for that "Bad Moon Rising" at 9:35 pm CDT