Several studies have examined specific characteristics of firms while attempting to explain highly skewed firm size distribution and the presence of extreme values. This study adapted the quantal response statistical equilibrium model of boundedly rational firms and used firms’ probabilistic decision-making to infer the equilibrium relative size distribution. The theoretical model complements conventional and entropy-based concentration measures. The study presents an adaptation for business firms in the United States while investigating firms’ expansion decisions, aspired sizes, and responsiveness to opportunities.