Based on the achievements acquired by scholars worldwide in research on the relativity between R&D investment and firms scale, this paper establishes a distinctive and synthetic index, called "Relative Elasticity of R&D Expenditure". Thereby, this paper analyses deeply the relation between R&D investment intensity and scale of firms in USA, China and Wuhan, compares their similarities and differences of R&D deportment in large and medium sized firms, and then draws strongly explanatory conclusions.