A free-piston engine linear generator (FPEG) system is used to generate power by reciprocating the mover of a linear generator with the combustion of an engine and the spring thrust. The high-frequency driving of the mover improves the thermal efficiency of the engine. However, the thrust for starting engine ignition increases at high frequencies; hence, a relatively large generator is required. This paper clarifies the effects of generator downsizing associated with utilizing a lower starting thrust than conventionally used thrust of the spring resonance and the thermal-efficiency improvement realized through high-frequency driving.