Adipose tissue expands to accommodate increased lipid through hypertrophy of existing adipocytes and by initiating differentiation of preadipocytes. The capacity of adipose tissue to expand is critical for accommodating changes in energy availability, but this capacity is not an unlimited process and likely varies between individuals. We suggest that it is not the absolute amount of adipose tissue but rather the capacity of adipose tissue to expand that affects metabolic homeostasis. Here we highlight examples of disease states and transgenic animal models with altered adipose tissue function that support this hypothesis and discuss possible mechanisms by which altered adipose tissue expandability impairs metabolic homeostasis.