This paper proposes a simple terminology for understanding and dealing with two current phenomena; we suggest calling them heavyweight and lightweight IT. Heavyweight IT denotes the well-established knowledge regime of large systems, developing ever more sophisticated solutions through advanced integration. Lightweight IT is suggested as a term for the new knowledge regime of mobile apps, sensors and bring-your-own-device, also called consumerisation and Internet-of-Things. The key aspect of lightweight IT is not only the cheaper and more available technology compared with heavyweight IT, but the fact that its deployment is frequently done by users or vendors, bypassing the IT departments. Our theoretical lens is generativity, the idea that complex phenomena arise from interactions among basic elements. In the context of IT, generativity helps to explain the creative potential of flexible digital technology for knowledgeable professionals and users. The research questions are: how is generativity different in heavyweight and lightweight IT, and what is the generative relationship between heavyweight and lightweight IT? These questions were investigated through a study of four cases in the health sector. Our findings show that (i) generativity enfolds differently in heavyweight and lightweight IT and (ii) generativity in digital infrastructures is supported by the interaction of loosely coupled heavyweight and lightweight IT. The practical design implication is that heavyweight and lightweight IT should be only loosely integrated, both in terms of technology, standardisation and organisation.