The high-frequency transformers used for isolated DC-DC converters tend to have large volume compared with the other components. Therefore, increasing the power density of the transformer is indispensable for increasing the power density of the DC-DC converter. Reducing the dead space of the transformer is one of the effective solutions. It is well-known that the thinly extended core structure reduces the dead space around the transformer formed by the windings protruding to the outside of the core. However, it has not been clarified how much the thinly extended core structure affects the reducing power loss of the transformer. In this paper, the effect of introducing the thinly extended core structure into the transformer is quantified by the optimization of the convertible design parameters of the transformer with systematic round-robin calculation. The result revealed the volume reduction effect by the thinly extended core was greater than the simply dead space reduction effect.