Aniseikonia occurs when the perceived image of each eye is different in sizes. This is often because each eye has a different prescription. For example, one eye might be significantly more short-sighted (myopia) or long-sighted (hyperopia) than the other, so one eye is essentially 'seeing' differently to the other.
The brain needs a similar image from each eye to be able to merge the two together to form a single, complex 3D image. If the image size from each eye is too different, the brain won't be able to merge the two together to form single vision, and hence double vision occurs. Because the brain can't process doubled vision, it will 'turn off' or suppress the image from one eye (usually the image that is the most abnormal). This can result in amblyopia or a lazy eye.