for my index of 48 million documents YaCy uses about 100GB.
But my Setup ist not representative, because I limit my index very much:
I use a Limitation of number of references per word of 10.000, wich is very low, but having a high amount of RWIs uses much RAM
I don't use any Web Structure Index.
Even this is a cool feature of YaCy, this takes a lot of resources. For me the benefit does not compensate the cost, but perhaps I should give it another try.
I limit the token count in the solr-schema.
Doing so limits the amount of space used by the Index by cost of loosing the full information of the documents.
As you can see I gave up a lot to get my Index that compact.