Set a target weight for the model and apportion that weight out to each major piece (fuse, LG, engines, wing, etc. -- don't forget the finish and details). Then as you build, weigh everything that goes in, and when you can weigh the pieces as they get built. I find that weighing each sheet of balsa before I use it does marvelous things -- heavier sheets get put back on the shelf when it's not a load bearing part, only to get pulled off the shelf when it's a long piece that needs to carry some weight.
Your ship will still be heavy for its size because it's scale, but it'll be lighter because you were forcing yourself to think about weight throughout.