A dual capacity compressor, which can maintain a fixed eccentricity and stable operation, even when the compressor is rotated in both the regular and reverse directions in order to provide multiple compression capacities, is provided. This dual capacity compressor implements an improved key member and associated key member fitting parts to inhibit relative motion of the crank pin and the eccentric sleeve during operation, so as to reduce or eliminate a destabilizing effect due to centrifugal forces on the eccentric sleeve and external forces applied through the connecting rod.