If you're having trouble with your computer handling such a large file, it often is more than just file size to consider. The detailed topography is what is causing your computer to struggle. You want to turn off of much of it as possible so your computer is not always trying to manage all of it at one time unnecessarily. So here's my suggestion. When working on really difficult topographies, it can also be helpful to split it up into pieces and group each piece then move it on to a different layer. This was you can turn off the layers that have the pieces of topography you are not working on right at that moment. This makes life much easier on your computer. Like I said, this won't make the file smaller, but it will make it more manageable. Good luck!
Chris