Large adipocytes increase vesicle-mediated lipid release and promote breast cancer malignancy.

Primary adipocytes exhibit striking variability in size, yet the functional consequences of adipocyte hypertrophy remain unclear due to insufficient experimental approaches to control for cell size. Here, we establish methods to culture large and small primary adipocytes isolated from the same adipose depot, enabling size-resolved analyses independent of systemic obesity. Using transcriptomic, lipidomic, and functional profiling across two mouse models of obesity, as well as human clinical samples, we show that adipocyte size-rather than body weight-drives distinct phenotypic cell states. Notably, large adipocytes increase extracellular vesicle-mediated lipid release. In coculture assays, this shift enhances lipid uptake, migration, and proliferation of breast cancer cells through fatty acid oxidation. Consistent with these findings, individuals with larger mammary adipocytes exhibit elevated fasting triglycerides independent of body mass index. Together, our results identify adipocyte size as a key determinant of adipose tissue function with implications for both metabolic disease and cancer progression.