The discharge flow in a cylindrical and a rectangular silo using both monodisperse and bidisperse mixtures of spherical glass beads is studied experimentally. The flow rate is measured using a precision balance for a large range of particle diameters, size ratios, and outlet diameters. A simple physical model is proposed to describe the flow of bidisperse mixtures. It gives an expression for the flow rate and predicts that the bulk velocity follows a simple mixture law. This model implies that a mixture diameter cannot be simply defined. Moreover it is shown that bidisperse granular media allow for the transport of coarse particles below their jamming conditions.