Personally, I use a large Honda (ST1100) and things fit very nicely in the panniers.
I would seriously discourage any use of bungees/ straps etc. I once saw the consequences of a holdall that escaped its straps and went into the rear wheel of a m/c.
Remember that weight limits for motorcycle luggage is usually quite small, it is inevitable that you will have to reduce the load if you are doing an overnighter etc!
Leading egde!