Participatory Design (PD) has traditionally been committed to extensive interaction between developers and situated users to mitigate the disempowering consequences of computerization, such as the deskilling of labour workers. However, the widespread adoption of off-the-shelf software and the emergence of complex information system architectures with interdependencies across user groups and organizations challenge the applicability of traditional custom PD. Pressure is put on PD to scale with initiatives that span an increasing number and distribution of heterogeneous settings, developers, users and uses over time. In this article, we follow a PD project that started out in post-apartheid South Africa more than two decades ago. The project, which centres on the development of a software product for decentralised public health care management, has since grown into a venture with a significant footprint in the Global South. In order to problematise the scaling of key aspects of PD, such as the politics of design, the nature of participation and participatory design techniques, we first review extant literature and develop a classification of four different types of PD with respect to scale. We then apply the typology to our empirical case to discuss PD in relation to architectural traits at different stages of project scale. We contribute to PD literature by addressing the exploratory research question: What role does architecture play in large scale PD? Specifically, the study highlights how an emergent platform architecture and its surrounding ecosystem co-constitute a platform for participation in design