Magnification of concave mirror = - v/u
= negative for a real inverted image, the image is smaller when object is farther than center of curvature and more than that of object when object is between center and focus.
= positive for an erect virtual image, with m > + 1 as image is bigger than object, when object is kept inside focal length.
magnification of a convex mirror = -v/u
= always positive as virtual images are formed and erect. These are always smaller than object size. Hence magnification is less than 1.
From the above 3 cases, magnification of +1/3 in a mirror is only possible for a convex mirror.