It is quite big, the advantage here is two up riding. The wife and I can go somewhere casually, put both of our helmets in the trunk and a jacket in each side pannier. That leaves two people without gear to haul around when we walk around at our destination. Hard to understate how nice that is.
If you want a bit smaller though, I would go with the Yamaha OEM top boxes for this rack. They have a smaller ones (35L and 50L i think) that are meant to pair with this rack. They look decent, and you can get a backrest with it.