For large-scale category systems, such as Directory Mozilla, which consist of tens of thousand categories, it has been empirically verified in earlier studies that the distribution of documents among categories can be modeled as a power-law distribution. It implies that a significant fraction of categories, referred to as rare categories, have very few doc-uments assigned to them. This characteristic of the data makes it harder for learning algorithms to learn effective de-cision boundaries which can correctly detect such categories in the test set. In this work, we exploit the distribution of documents among categories to (i) derive an upper bound on the accuracy of any classifier, and (ii) propose a ranking-based algorithm which aims to maximize this upper bound. The empirical evaluation on publicly available large-scale datasets demonstrate that the proposed method not only achieves higher accuracy but also much higher coverage of rare categories as compared to state-of-the-art methods.