One cause of significant anisometropia and subsequent aniseikonia has been aphakia. Aphakic patients do not have a crystalline lens. The crystalline lens is often removed because of opacities called cataracts.
The removal of the crystalline lens, often due to cataracts, can lead to anisometropia and aniseikonia, which is a condition where there's an uneven refractive power between the eyes.  Aphakia, the absence of a crystalline lens, is a major cause of this issue.