WE know that a convex lens forms a virtual and erect image when the object is kept between the optical center and the principal focus of the convex lens.
THIS virtual image is much larger than that of the object and hence the magnification ,
MORE over when the object is at focus then the image formed at infinity is larger than the object ,i.e a magnified image is formed and
More over if the object is also between C and F then also a magnified image is formed.
SINCE ALL THESE CAPABILITIES ARE NOT PERSUADED BY OTHER LENSES ,i.eCONCAVE LENS ,HENCE ONLY CONVEX LENS IS USED IN A MAGNIFIER.