I wouldn't expect that it's a matter of the bike being heavier. The frontal knee pain usually indicates a seat that is a little too low, or you might be pushing harder on the forward part of the downstroke, possibly because of the more upright position. Any time I ride a bike I'm not used to I know that I'm bound to get a few odd aches and pains, just because my body hasn't had time to adapt to that exact riding position yet. The problem with touring bikes is they make you want to ride slow, and that sometimes leads to bad form which leads to problems. You have to find a happy medium.