Vanvector

Can anyone comment on average file sizes for landscape designs. I transfer over autocad base files from architect, so i start off with a file about 1mb and then begin working on the file but by the end of the concept plan i seem to have a 15-20mb file. This file is slow to work with on my system. Is this a normal file size ? I have quite a few classes, a couple of deisgn layers and are using sheet layers also. thanks