MSAA cost is more affected by tiny triangles on screen, so usually you can make LOD’s to turn many tiny triangles in the distance into fewer large ones. This usually helps with aliasing too, and reduces vertex cost.
The reason this happens is that MSAA increases the pixel footprint of a triangle which needs to be shaded. So every triangle affects more pixels. For a large triangle it might only be 10% more pixels, but for a small triangle it can be 400% more pixels.