Why does it look so fucking big sometimes (like when it is low on the horizon, evening time) but then small other times (middle of night)?

why?

optical illusion.
next full moon you see very low near building and the horizon, extend your hand out all the way and compare the size of the moon to one of your finger nails (which ever size is closes to that of the moon)
then when you see high in the sky preform the same comparison
youll see its the same size.

vouch. its all in your head, same thing happens at sunrise, sunset.

http://www.howstuffworks.com/question491.htm

I love howstuffworks. =]

It answers the question more adequately.

The world is a questionable place, so great.