Oxford sports soft panniers - they're smaller but you'll be surprised what you can get in them, and your passenger will still sit comfortably on the bike. Also, if you have a luggage rack, you can get big square tailpacks which we use for a double douvet and a blanket (sleeping bags are not very intimate

. Also, the Baglux tankbags are great. They expand so you can fit all the stuff you need on the day, money, map, **** etc, I also put my toiletries in mine. On the pic below, you can just about see the tailpack on Dave's red fireblade and his Oxford panniers. He's got other stuff on the pillion seat too (it was a 2 week holiday!)
Incidentally, with tank bags it's worth bearing in mind that some have magnetic pads which won't work on the plastic coated Triumph tanks! Baglux do coloured tank covers which have fixings to clip your bag onto. Genius!