Population abundance is negatively related to body size for many types of organisms. Despite the ubiquity of size–density scaling relationships, we lack a general understanding of the underlying mechanisms. Although dynamic models suggest that it is possible to predict the intercept and slope of the scaling relationship from prior observations, this has never been empirically attempted. Here we fully parameterize a set of consumer–resource models for mammalian carnivores and successfully predict the size–density scaling relationship for this group without the use of free parameters. All models produced similar predictions, but comparison of nested models indicated that the primary factors generating size–density scaling in mammalian carnivores are prey productivity, predator–prey size ratios, and consumer area of capture.