Easy, you fill all the empty cavities in the fuse and wings with helium! I think manufacturers should start putting that statement in the specs. I just finished a Creek hobbies Staudacher. I think they are supposed to weigh around 10.5-11.5 lbs. If you weighed all the components separately, you might hit that weight. But they don't mention the pound of nose weight you have to add to make the thing balance.

Really, the servos should be mounted in the wing bay of these planes instead of the tail. I assume the manufacturer know this. Why don't they build the kit for this configuration?