There are many times that you don't need to retrieve the documents from a graphLookup, you only want the number of resulting documents after performing the graphLookup stage, you are throwing away capacity in the process, if you after the graphLookup are going to do a $size operation.

If the graphLookup could resolve in a number being this number the count of the resulting documents, could be way more easy to retrieve the needed data without having to use so many resources.