I think, if you still feel like reducing the size of your level in memory, simply free parts of the level, and be ready to load them later. Freeing the actual meshes and textures will free up more RAM for sure. It'll be complicated because you'll definately want to reload the parts of your level before your player sees them again, and you may want to do that gradually instead of pausing your game to reload data. But all this is just ideas to consider.