Larger than expected apk size when adding prefab instances.

Hi there. I am trying to make a mobile game (currently just android) and have been testing the size increase when adding more and more prefab instances. I thought that the increase would be very small <1k since they are just identical copies but the size increase has been 60k for a very simple model with 512k texture (not that that should make a difference) - At this rate it would make it unfeasible to add things like trees and plants which are very numerous as the app size will balloon. Is there something which i am doing wrong here or are these size increases just something which are unavoidable? I just cant seem to get my head around the source of this memory usage. many thanks in advance for any help.