We present an experimental study of drainage in two-dimensional porous media exhibiting bimodal pore size distributions. The role of the pore size heterogeneity is investigated by measuring separately the desaturation curves of the two pore populations. The displaced wetting fluid remains trapped in small pores at low capillary numbers and is swept only above a critical capillary number proportional to the permeability of the big pores network. Based on this observation, we derive a simple criterion for phase trapping based on the balance of viscous to capillary forces. Numerical implementation of this theory in a pore network model quantitatively fits our experimental results. This combination of approaches demonstrates quantitatively the influence of geometrical heterogeneities on drainage in porous media.