D&C and D&E are not that different; yes, technique and risks are slightly different, but it's kinda moot point.
The specific difference is using a curette
(D&C) and there is a limit to the size of products of conception (POC) that can be evacuated (think vacuum hose and bowling ball). The cervix can only be dilated so much mechanically. D&E uses instruments to fragment the POC thus has no limit.