The reason that Maximum Magnification of any lens, including a 1:1 Macro lens, is at the Minimum Focus Distance is because that is as close as you can be to the subject and still be able to bring the subject into focus.
What happens when you get closer than the MFD of any lens? The lens is unable to focus at that distance, because it is closer than the minimum distance that the lens can focus. Your subject will be larger in the frame, but it will also be blurry and out of focus.
What happens when you get further away than the MFD from your subject? When you move further away from the subject the subject will appear smaller in your image. If a lens' maximum magnification is 1:1, it can only achieve this magnification at the MFD. If you move back you will no longer have 1:1 magnification of your subject. The further back you move, the smaller the relative size of the subject in the image. You will also need to refocus on the subject that is now further away from your camera. Since you are further away from your subject, thus rendering the subject smaller in your image, you will lose some detail of your subject.