(Image credit: Galileo project & NASA.)
Why's this? The Moon orbits in a giant ellipse around our Earth, where sometimes it's at its closest to us and sometimes it's at its farthest. And most of the time, this is what's responsible for how large the Moon appears.
Not only that, but some perigees -- the point of the Moon's closest approach to Earth -- are closer to us than others! These slight variations are due to the gravitational forces of other objects (mostly the Sun) in our Solar system, and it causes the actual perigee and apogee (when the Moon is farthest from Earth) distances to vary, periodically, over time!
(Image credit: this thread... good luck understanding it!)
In addition to that, the Moon is exactly 100% full only one instant a month, and that moment is very unlikely to line up with the exact moment of perigee. In other words, sometimes we see the Moon full near perigee, sometimes near apogee, and most of the time somewhere in-between.
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