The importance of the relationship between size and defect proneness of software modules is well recognized. Understanding the nature of that relationship can facilitate various development decisions related to prioritization of quality assurance activities. Overall, the previous research only drew a general conclusion that there was a monotonically increasing relationship between module size and defect proneness. In this study, we analyzed class-level size and defect data in order to increase our understanding of this crucial relationship. In order to obtain validated and more generalizable results, we studied four large-scale object-oriented products, Mozilla, Cn3d, JBoss, and Eclipse. Our results consistently revealed a significant effect of size on defect proneness; however, contrary to common intuition, the size-defect relationship took a logarithmic form, indicating that smaller classes were proportionally more problematic than larger classes. Therefore, practitioners should consider giving higher priority to smaller modules when planning focused quality assurance activities with limited resources. For example, in Mozilla and Eclipse, an inspection strategy investing 80% of available resources on 100-LOC classes and the rest on 1,000-LOC classes would be more than twice as cost effective as the opposite strategy. These results should be immediately useful to guide focused quality assurance activities in large-scale software projects.