I just created a scene by multireplicating 67 trees and the file size was 2 meg. Are you creating 67 unique trees? If so, do you need to? With the sheer amount, individualizing trees may be unnescessary. Simply rotating a duplicate tree will fool the eye into believing it to be different from the one next to it. I've created very convincing jungle scenes using this method and the end product was only 7 meg.