This paper experimentally investigates the DC breakdown characteristics of oil-impregnated pressboard with respect to specimen thickness and layer configuration. Breakdown tests were conducted ten times for each condition on single specimens of 1T, 2T, and 3T thickness (nominal 1, 2, and 3 mm) and on a stacked specimen consisting of three 1T sheets, using a sphere-sphere electrode arrangement with a diameter of 4 mm. The results were analyzed with a two-parameter Weibull distribution in accordance with IEC 62539. As the thickness of the single specimen increased from 1T to 3T, the breakdown field based on the mean electric field, normalized to the 1T condition, decreased from 1.000 pu to 0.601 pu, a reduction of 39.9 %, while the shape parameter decreased from 37.4 to 10.8. When the field utilization factor obtained from finite element analysis was taken into account, however, the reduction was 11.3 %, indicating that a substantial part of the observed decrease originates from the increased field non-uniformity associated with the electrode geometry. The corrected reduction still exceeded the 4.9 % predicted by the enlargement law. In contrast, for the same nominal total insulation thickness of 3 mm, the stacked specimen exhibited a breakdown field of 0.684 pu, which is 14 % higher than that of the single specimen of equal thickness, and the 90 % confidence intervals of the scale parameters did not overlap, confirming that the difference is statistically significant. Since the two conditions have essentially the same stressed volume, this difference is attributed not to the number of defects but to the layer configuration of the insulation. These results suggest that securing the total insulation thickness alone does not guarantee the required breakdown strength in DC insulation design, and that the layer configuration should be considered as a design parameter.