One does not make a capacitor, two is a party so which one is it, Cgs and Cgd? As far as I could recall the capacitance does not varies with voltage, it is tagged to the aspect ratio. However, maybe you should rather consider the effect of these capacitance at your frequency of interest.
Gate capacitance is a function of what region of operation the MOS is in-Accumulation, weak inversion or strong inversion. The cap is close to WLCox in accumulation, gradually decreases as the MOS enters weak inversion. From weak inversion to strong inversion there is a sharp increase. The cap value comes back close to WLCox.