The two Fanaroff & Riley classes of radio galaxies are defined by radio morphology, and the distinction may arise from a combination of environmental conditions and central engine power. Several studies have suggested that powerful, FR II, radio galaxies are on average 0.5 mag fainter optically (i.e., are smaller) than weak, FR I, radio galaxies; Although other optical properties are the same, and are indistinguishable from those of normal elliptical galaxies. Here we discuss the host galaxies properties of FR I and FR II sources and suggest a possible explanation of the observed difference in average optical luminosity. In particular, we discuss how the optical plus radio luminosity function can induce a difference due to selection effects.