A magnetic sail is spacecraft propulsion that produces an artificial magnetosphere to block solar wind particles and thus impart momentum to accelerate a spacecraft. In the present study, the authors conducted two-dimensional particle-in-cell simulations on small-scale magnetospheres to investigate thrust characteristics of a magnetic sail and its derivative, magnetoplasma sail, in which the magnetosphere is inflated by an additional plasma injection. As a result, the authors found that the electron Larmor motion and the charge separation become significant on such a small-scale magnetosphere and the thrust of the magnetic sail is affected by the cross-sectional size of the charge-separated magnetosphere. The authors also reveal that the plasma injection, on the condition that the kinetic energy of plasma is smaller than the local magnetic field energy (approximate to 10-3), can significantly inflate the magnetosphere by inducing diamagnetic current in the same direction as the onboard coil current. As a result, the magnetoplasma sail thrust is increased effectively by an additional plasma injection: the magnetoplasma sail thrust (15N/m) becomes up to 7.5 times larger than the original thrust of the magnetic sail (2.0N/m). In addition, they found that the thrust gain of the magnetoplasma sail, defined as magnetoplasma sail thrust/(magnetic sail thrust+plasma injection thrust) becomes up to 2.2.