Carrying 8.1 lbs. around all day makes a difference and most of that weight is up high which compounds the problem. It makes a huge difference for people like me who are vertically challenged.
True, but I'm used to 150+ mile days in the eastern woods on a XR650R. Compared to that the '05 WR450 feels like a 250 two stroke. The '05 WR hides it's weight well. I had a chance to spend a few days on a well set up '04 WR and it didn't feel much better in the tight stuff than the big XR. The small changes they made to the '05 made a BIG difference. Still, none of the big off road 4 strokes are anywhere near what I'd call a lightweight bike.
If weight is real important to you then you can't do much better than that KTM 300 you're riding now.