It can be too large to fit in the case with the gasket installed, but still smaller than the physical opening in the case. That's why he has to glue it, because it won't press-fit into the gasket.
If you find a crystal that fits the gasket let us know, the rep community has been looking for one forever. Otherwise, having a crystal glued in is your only option. If it's done carefully and correctly, it'll be almost as good as new.