Recently, the layout optimization of the secondary coils for wireless power transfer systems has been developed using the fixed grid (FG) representation. Although this method can effectively and efficiently determine the optimized coil design under the given conditions, a distinct discrepancy exists between the evaluated coil mass (i.e., simulation data) and the physical coil mass (i.e., experiment data). In this paper, the FG-based coil layout optimization is revisited based on the smooth boundary (SB) representation in order to resolve the inherent drawbacks of the FG representation. Induced voltage, electromagnetic field strength, and coil mass, which have been previously derived in the FG representation, are reformulated in terms of design variables (i.e., a reference turn and relative turns in this paper) in the SB representation. Through the layout optimization, the optimized layouts of the secondary coil are determined in order to minimize the coil mass while satisfying the constraints for induced voltage and magnetic field strength; then, they are postprocessed in order to obtain a smooth, elaborate boundary. The experimental validation demonstrates that the proposed SB-based approach outperforms the previous FG-based method in terms of coil mass and boundary representation.