Why can't I find a large diameter magnifier with a large magnification?

This has to do with the physical limitations of a lens. Note from the magnifying power (MP) equations above, that the MP and EFL have a reciprocal relationship. In order to increase the MP, the EFL needs to be decreased. Because the approximation from the Lens Maker's Equation for a DCX lens is EFL=R1=R2, the radius of curvature (R) also needs to be decreased. In order to decrease the radius, the lens must be physically reduced in size. The result of a smaller lens diameter is a smaller field of view. Accordingly, as the magnifying power increases, the size of the magnifier is reduced.