The amount of stuff does seem to be a factor for sure. It was worst on the side that my sleeping bag was in. The bag completely filled that pannier. The other side, which tended to have an extra roll or two at the top, didn't have water nearly as often. The sleeping bag side was somewhat often. This was on a tour that ended up with probably 12 days of rain, often raining for a good portion or several times per day. If you weren't on a really rainy tour, I wouldn't worry about them at all. Even then my stuff never really got wet. There was just a little water that would get in at the top. If I had had clothes or something at the top of that bag, they would have gotten a little damp. My bag has a water resistant shell, so the water didn't soak into it anyway.