A mean-centric view of population, whereby a change in the mean level of a health parameter at the population level is assumed to result in uniform change across the distribution, is a core concept in Geoffrey Rose’s concept of the “population strategy” to prevention and public health. This idea also has a critical role in Rose’s observation that those considered abnormal or sick (the rightward tail of the distribution) and those considered normal (the center) are very closely related, and that true preventive medicine must focus on shifting the “normal” or “average”. In this Perspective we revisit these core tenets of Rose’s concept of population health and prevention, after providing an overview of the key concepts he developed. We examine whether these assumptions apply to population changes in body mass index (BMI), and show that there is considerable evidence of widening of the BMI distribution in populations over time. We argue that with respect to BMI, the idea of using statistical measures of a population solely based on means, and the assumption that populations are coherent entities that change uniformly over time may not fully capture the true nature of such populations. This has important implications for how we assess and interpret the health of populations over time, with implications for the balance between universal and targeted strategies aimed at improving health.