Aniseikonia - different size of retinal images
A larger retinal image in one eye compare to the other eye makes it hard for the brain to melt the images together. This can cause diplopia.
When the retinal image in the right eye does not match the retinal image on the left eye in size, we call that aniseikonia. This could be a result from a trauma or complication after cataract or refractive surgery. The far most common reason is anisometropia, when the refractive error is much different in one eye than the other.
Too much difference in refractive error - anisometropia
When correcting refractive errors with glasses, there is always a magnification or a minification present. Plus lenses (correcting hyperopia) magnifies the images and minus lenses (correcting myopia) minifies the image. When having different powers for right and left eye, this can cause problem fusioning the images together.
Normally we talk about that problems can occur if the difference is around 2-3 dipters but this is very much individual. It is possible with different techniques to correct this in glasses. Normally, wearing contact lenses also reduces this problem.
Prismatic errrors due to different powers
In the periphery of a lens, there is a prismatic effect. If you wear glasses with different powers, there is no problem when looking through the center of the lens. But when looking down or up we use the periphery and experience the prismatic effect. Because of the different powers in the lenses, the prismatic effect is different. This can cause eye strain or even diplopia. This is important to know because this can explain difficulties looking at different directions.