A Forest object on "Custom Edition" mode always takes more memory, because it have to store the position of each tree. Instead, trees on "Generate" mode are not saved with the scene, but created dinamically. If you want to remove trees, sometimes is better to use Exclusion area splines.
However, i going to send you an username and password to our FTP server, so you can upload the scene. See it on the private messages.
About the materials, we suggest to use always "Forest Material 3" for Forest objects, because it's needed for a lot of features (shadows, random tint and more).