We investigate multiphoton states generated by high-gain optical parametric amplification of a single injected photon—polarization encoded as a qubit. The experimental configuration exploits the optimal phase-covariant cloning. The output state of the apparatus is found to exhibit the quantum superposition property of mesoscopic multiphoton assemblies involving about 300 photons. This work represents an experimental advance toward the test of several fundamental quantum processes in mesoscopic or macroscopic frameworks