It does change during the lunar cycle. daily changes would have to be an optical illusion from the atmosphere.
It isn't always in the same phase because the relationship between the earth, moon and sun is different every orbit, as we go round the sun and the moon orbits the earth.
Once in a while you get a full moon right when it's as close as it gets to the earth, and the moon looks HUGE.
Edit-
The actual time it takes the moon to orbit the earth and the time it takes to go from one full moon to another isn't the same, because we've gone approximately 1/12 of the way around the sun while the moon makes an orbit, there's about an 8% difference, so it takes a couple days longer before the moon appears to havecompleted an orbit.
I believe the moon actually orbits the earth 13 times a year, but it appears to only be 12 because we go all the way around the sun in the meantime.