When does Cartesian genetic programming minimize the phenotype size implicitly?

A new method is proposed to minimize the number of gates in combinational circuits using Cartesian Genetic Programming (CGP). We show that when the selection of the parent individual is performed on basis of its functionality solely (neglecting thus the phenotype size) smaller circuits can be evolved even if the number of gates is not considered by a fitness function. This phenomenon is confirmed on the evolutionary design of combinational multipliers.