Hi Barb ~ Yes, there is a downside to too-large panniers. If you tour with only rear panniers, it will really degrade your bike handling. I was testing out some large Arkel panniers on a two-day "shakedown" tour last fall, and found that my bike handled fairly poorly because of too much weight on the rear and not enough on the front. The front end was real squirrelly. I ended up going with a smaller size pannier in the rear and a small set of front panniers, which balanced the weight on my bike far better. In fact, it handled as if there were no panniers at all on it! I've read that you should actually put more weight in your front panniers than in your rear, since a larger % of your own body weight is already on the rear of the bike.
Also, if you have a bike with short chainstays and you have largish feet, your feet might actually hit large rear panniers. I had this happen even though my feet aren't that large, because I have very short chainstays (my touring bike is a Bike Friday).
Other than that, of course you want to pack as light as possible, and large panniers that encourage everything-but-the-kitchen-sink type packing will only make you curse on every climb. I think you'd be better off with front and rear panniers in smaller sizes to better balance your load.
Good luck!
Emily
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow