>>95928153
Because of scale inconsistencies stemming from compositing errors/choices in the OT movies. The canonical size of the Neb-B is based on the scene at the end of Empire Strikes Back with Luke and Leia at the window of the medical bay. But even within that scene, the scale is clearly wrong, as the miniature of the Millennium Falcon docked to the frigate is far too small relative to the window, suggesting a larger sized Neb. And then in Return of the Jedi, the Neb is practically Star Destroyer sized when compared to a Corellian corvette in the foreground.