Thermionic power generation is a safe and clean energy source that allows for converting heat into electrical energy using thermionic electrons. The miniaturization is an advantage of this technology that led to the recent development of micro-gap thermionic power generators. In this work, thermal contact resistances between the micro-gap insulators and the emitter as well as between the micro-gap insulators and the collector are measured. A thermal resistance of 48.6 K/W is obtained by downsizing the insulators until 60 × 45 μm2 of contact area with the emitter, demonstrating a high impact for decreasing the micro-gap conduction heat loss density from the emitter to the collector from 28 W/cm2 (theoretical value obtained without considering contact resistances) to 5.6 W/cm2. Downsizing the contact area between the insulators and the emitter from 320 × 300 to 60 × 45 μm2 leads to an increase of the power conversion efficiency from 9.1 × 10−5 until 1.5 × 10−3.