A model is presented that explains the mass loss rate in a compartment as a function of scale. The effect of ventilation is included in the model by the inclusion of the ambient oxygen concentration in the lower layer that results due to vent mixing. The model is executed in BRI2002, a zone model, capable of computing species and thermal conditions in the upper and lower compartment gas layers. Computations show good agreement with two different scale-down compartment fires for liquid fuel. The results can accurately distinguish scale effect from these experiments and allow us to focus on fundamentals of fire phenomena.