There’s not really an absolute outline of a mesh. The outline (if you mean the “external borders” of the mesh) depends on the camera view.
For a sphere, it could seem the outline is always the same because of the symetries, but think of it when textured: you won’t see the same thing depending on the camera view.
You can decrease the number of triangles by lowering the number of segments used in the create sphere method (if you created the sphere with this method), but that’s about it…