Ok if you're just throwing it up on the wall the lens is not perfectly perpendicular to the wall. So you have it pointing to the side that is larger. Just turn the projector a little bit to the side that is smaller until they are even.
Edit: I guess technically the lens should be parallel to the wall and the center of the shaft of light coming out of the projector should be perpendicular to the wall.
What is happening is that the light on your left side has to travel farther to get to the wall therefore it is larger. If you were to draw a triangle where one point is the projector and the line opposite that point is the wall that would be the right side of your image. Now extend the lines going from the projector to the wall and draw another wall... this is the left side of your image and your second wall is longer than your first wall.
This is the best way I could think to explain what is going on. I hope it helped.
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