In gravity-driven silos, grain discharge increases with outlet size. Here we experimentally show that fluid-driven granular discharge through a horizontal outlet behaves in the opposite way: at fixed imposed flow rate, smaller outlets can transport more grains. This behaviour arises from a mobilisation threshold at which fluid stresses must overcome friction associated with the submerged weight of the grains. Reducing the outlet size increases the fluid velocity and therefore the driving stress, enabling greater grain transport. Geometric confinement introduces an additional resistance that further modifies this behaviour. We show that confinement both raises the mobilisation threshold and reduces the maximum grain fraction of the flow. A unified model capturing these effects reproduces the observed dependence of grain transport on outlet size.