Software effort estimation is still a significant challenge for software management. Although Functional Size Measurement (FSM) methods have been standardized and have become widely used by the software organizations, the relationship between functional size and development effort still needs further investigation. Most of the studies focus on the project cost drivers and consider total software functional size as the primary input to estimation models. In this study, we investigate whether using the functional sizes of different functionality types, represented by the Base Functional Component (BFC) types; instead of using the total single size figure have a significant impact on estimation reliability. For the empirical study, we used the projects data in the International Software Benchmarking Standards Group (ISBSG) Release 10 dataset, which were sized by the COSMIC FSM method.