Achieving system-level, sustainable 'scale-up' of interventions is the epitome of successful translation of evidence-based approaches in population health. In physical activity promotion, few evidence-based interventions reach implementation at scale or become embedded within systems for sustainable health impact. This is despite the vast published literature describing efficacy studies of small-scale physical activity interventions. Research into physical activity scale-up (through case-study analysis; evaluations of scale-up processes in implementation trials; and mapping the processes, strategies, and principles for scale-up) has identified barriers and facilitators to intervention expansion. Many interventions are implemented at scale by governments but have not been evaluated or have unpublished evaluation information. Further, few public health interventions have evaluations that reveal the costs and benefits of scaled-up implementation. This lack of economic information introduces an additional element of risk for decision makers when deciding which physical activity interventions should be supported with scarce funding resources. Decision-makers face many other challenges when scaling interventions which do not relate to formal research trials of scale-up; Methods: To explore these issues, a multidisciplinary two-day workshop involving experts in physical activity scale-up was convened by the University of Newcastle, Australia, and the University of Ottawa, Canada (February 2019); Results: In this paper we discuss some of the scale-up tensions (challenges and conflicts) and paradoxes (things that are contrary to expectations) that emerged from this workshop in the context of the current literature and our own experiences in this field. We frame scale-up tensions according to epistemology, methodology, time, and partnerships; and paradoxes as 'reach without scale', 'planned serendipity' and 'simple complexity'. We reflect on the implications of these scale-up tensions and paradoxes, providing considerations for future scale-up research and practice moving forward; Conclusions: In this paper, we delve deeper into stakeholders' assumptions, processes and expectations of scaling up, and challenge in what ways as stakeholders, we all contribute to desired or undesired outcomes. Through a lens of 'tensions' and 'paradoxes', we make an original contribution to the scale-up literature that might influence current perspectives of scaling-up, provide future approaches for physical activity promotion, and contribute to understanding of dynamic of research-practice partnerships.