I don't think it is a matter of Cunners 'stepping up' at all or that he 'stands out more'.
For mine it is far more to do with the fact that Cunners and co get a better opportunity to show that what they are capable of when Jack isn't playing.
Cunners is much cleaner in tight and does not play the way JZ does, nor lose his feet so often, etc. They really shouldn't be compared as like for like some posters seem to.
As much as I admire Jack's uncompromising attack on the footy and obviously he is a key player going forward, it regularly doesn't allow our cleaner players in traffic to get hold of the footy and set up play.
For mine we have Cunners and Swallow as our extractors who are both excellent in close, Levi chipping in and Jack can play a utility style role as a mid as part of the rotation yes, but also get him into dangerous positions on the field to use his foot skills and strength in a one on one contest where he will create a difficult match up.