One thing I have noticed that makes file sizes very large is the assets themselves. Assets should be used in such a way that they are x-ref objects, so they are not actually present in the scene. One tree model can be huge, so if you merge it into your master scene it will make your file huge. But if you use it as an x-ref object, it does not become part of the master file, despite being visible in the file. I think you should be able to set the forest pack presets to x-ref, which should help with file size.