The crown should re-establish the proper form, function and aesthetics...
If the crown is big (over-contoured) this might cause further gum problems or even caries at the site where the crown join the tooth (finish line).
it makes it harder to obtain proper hygiene on that tooth (since it is harder to remove the bacteria that accumulate at the gum line on the tooth surface.
the crown should represent the form (shape and size) of previous lost tissues, even though in some situation we make it smaller to reduce the load on a tooth in special situations, but never larger !!!
I think it is advised to replace that crown with proper one, but the final judgment is after proper clinical examination.. ;)
If your dentist can not solve your problem, you may need to see a prosthodontist.