Educational interventions are often administered at scale in diverse settings as part of international development programmes. Their implementation is subject to a linear process that begins with finding out 'what works' at a local level, frequently through the use of randomised controlled trials, and continues with rolling out the intervention to the whole population at a national or even transnational level. This process often fails to consider the role cultural, political, and historical factors play in the perceived success of the local intervention, which can compromise both the impact and the ethics of at-scale implementation. To help address this issue, this paper argues for a definition of scalability that incorporates the ethics of the practice of scaling. It points to the potential of collaborative multi-sited ethnographic research to identify nuanced understandings of the different ethics systems endogenous to individual sites of implementation, in lieu of the universalising notions of ethics that are embedded in mainstream, linear notions of scalability. In so doing, it makes the case for multi-sited critical ethnography as a methodology of choice in researching the scalability of interventions in the context of development projects in the 'Global South'.