Yowser!!!
Would shortening the ferrule help? I'm not usually a fan if Mokume bolsters, but I really like how it picks up the warmth of the mammoth ivory.
I'm not an educated man, but it looks to me that if you are to taper the ferrule and perhaps some of the handle a bit of the weight would just go away. It's quite the handsome couple, and that pattern is groovy!
The Mokume is beautiful. Is it blasphemy to suggest maybe attempting to drill/file out the tang opening and replace with a lighter material, leaving the outside as is?