The increasing demand for software customization has led to the development of highly configurable systems. Combinatorial interaction testing (CIT) is an effective method for testing these types of systems. The ultimate goal of CIT is to generate a test suite of acceptable size, called a t-wise covering array (CA), where t is the testing strength. Pairwise testing (i.e., CIT with t=2) is recognized to be the most widely-used CIT technique and has strong fault detection capability. In pairwise testing, the most important problem is pairwise CA generation (PCAG), which is to generate a pairwise CA (PCA) of minimum size. However, existing state-of-the-art PCAG algorithms suffer from the severe scalability challenge; that is, they cannot tackle large-scale PCAG instances effectively, resulting in PCAs of large sizes. To alleviate this challenge, in this paper we propose CAmpactor, a novel and effective local search algorithm for compacting given PCAs into smaller sizes. Extensive experiments on a large number of real-world, public PCAG instances show that the sizes of CAmpactor’s generated PCAs are around 45% smaller than the sizes of PCAs constructed by existing state-of-the-art PCAG algorithms, indicating its superiority. Also, our evaluation confirms the generality of CAmpactor, since CAmpactor can reduce the sizes of PCAs generated by a variety of PCAG algorithms.