The question of how class size impacts on student learning has been debated for some time, not least because it has substantial financial implications for educational policy. The strength of this debate notwithstanding, results from numerous international studies have been inconclusive. The study from which this paper stems sought to conceptualise the effects of class size on teaching and learning processes, an under-researched area in this field. To do that, the study explored qualitative differences in learning processes in large and small classes, drawing on social learning theory and the conceptual framework of community of practice to elucidate any differences. The data set includes classroom observations and interviews with students and teachers in four pairs of large and reduced-size English language classes in four Hong Kong secondary schools. Findings show that students were more motivated and engaged in their learning in the small classes studied. They also showed that the characteristics of a community of practice were more evident in the smaller classes. This paper concludes that it is the conditions that foster the development of a community of practice that have a critical role in bringing about high quality learning and that these conditions tend to be more easily met in smaller classes although there is no necessary relationship between the two.