I'm afraid the more objects you use the bigger the scene size. It would be worth testing how the file size expands, check the file size after adding things to your scene each time.
I suspect however that you have so many objects that the 2Gb size is correct.
Unfortunatly, Bryce scene do take up a lot of space, especialy if you are using a lot of trees. If you are using a lot of trees and they are placed in the distance in the scene, then you could use 2d trees instead.
Hope that is of some help to you.