It might be the time of night your looking at the moon, and it's height in the sky. The moon always looks bigger when it's close to the horizon
Cool thing to try - take a dime, and you can hide the moon with it, no matter how big or small it appears in the sky.
Good explanation of the illusion on this page:
I don't consider myself an astronomy freak, just your friendly, local forest ranger type. But I hope this helps!