The 18-135 is quite big. It's why I bought the kit with the 18-55. That being said, ever since I got the camera I wished I had just gotten the 18-135. The 18-55 doesn't have enough reach for just having one lens with you, and although the 18-135 lens is bigger and heavier it isn't so much that it isn't worth the extra reach you get out of it. In my opinion anyway. I'd love to have a travel kit with the 18-135, and probably the 56/1.2or 35/1.4, I could ditch the 35 if I could add a x100 in to that kit.
Just my opinion..I'm sure other's have better thought process.