I often see commuters that use a Backroller or Frontroller that is only a third or half full, it is readily apparent that it is not full from the looks of it.

I can't comment on the City version. A friend of mine has them for touring but he fills them.

About three years ago on a tour I took a empty Backroller to a grocery store and mostly filled it up, it was not a problem carrying it on the bike mostly empty, the brackets still hold it to the rack.

As far as rolling it down further, you can do a few extra turns, but I am going to guess that you can't shorten the height of it by more than about five or six inches.