I think they covered this in high school geometry, if I can remember that long ago. The 1MOA refers to an angle. Think of the target as the base of a triangle. If you have multiple targets at different ranges, you have a series of similar triangles. The size of the base of the triangle is proportional to the distance (the "height" of the triangle.) So, if one triangle with a base defined by a 1MOA angle is twice as far away as another triangle, the base will be twice the size. For shooting purposes, as others have posted, it doesn't have to be any more complicated.