This paper presents a numerical study of multiphase flow in hydrocyclones with different configurations of cyclone size and spigot diameter. This is done by a recently developed mixture multiphase flow model. In the model, the strong swirling flow of the cyclone is modeled using the Reynolds stress model. The interface between liquid and air core and the particle flow are both modeled using the so-called mixture model. The solid properties are described by the kinetic theory. The applicability of the proposed model has been verified by the good agreement between the measured and predicted results in a previous study. It is here used to study the effects of cyclone size and spigot diameter when feed solids concentration is up to 30% (by volume), which is well beyond the range reported before. The flow features predicted are examined in terms of the flow field, pressure drop, and amount of water split to underflow, separation efficiency and underflow discharge type. The simulation results show that the multiphase flow in a hydrocyclone varies with cyclone size or spigot diameter, leading to a different performance. A smaller cyclone results in an increased cut size, a decreased pressure drop and a sharper separation, and, at the same time, an increased water split (thus worse bypass effect) and a more possibly unstable operation associated with rope discharge, particularly at relatively high feed solids concentrations. Both large and small spigot diameters may lead to poor separation performance. Accordingly, an optimum spigot diameter can be identified depending on feed solids concentration. It is also shown that for all the considered hydrocyclones, a better separation performance and a smoother running state can be achieved by the operation at a lower feed solid concentration.