The low ON-resistance of wide-band-gap (WBG) transistors is a key feature for efficient power converters, however, the anomalous loss in their output capacitance (COSS) severely limits their performance at high switching frequencies. Characterizing COSS-losses based on large-signal measurement methods requires an extensive effort, as separate measurements are needed at different operation points, including voltage-swing, frequency, and dv/dt. Furthermore, there is a practical trade-off in the maximum voltage and frequency applied to the device. In this letter we present a small-signal model to extract large-signal COSS-losses in WBG transistors. The charging/discharging COSS energy dissipation (EDISS) is presented as a function of effective COSS, series resistance RS, and switching-time tSW. Contrary to other methods, this technique directly leads to a general identification of COSS-losses at different operation points, revealing new insights on COSS-losses in WBG transistors, especially the dependence of EDISS on voltage and frequency. Based on the proposed approach, the issue of COSS-losses in enhancement-mode GaN and SiC transistors was assigned to the limited quality-factor of COSS. The precise characterization of COSS-losses proposed in this letter is essential for designing efficient high-frequency power converters.