This study analyzes the technical efficiency and the explanatory factors depending on the company size in a panel data over the period 2000-2010 in 26 Spanish industrial sectors three digits of the cnae (Clasificación Nacional de Actividades Económicas). The results achieved by the stochastic frontier model reveal the existence of two production frontiers for small and medium enterprises (Smes) and large companies (eg). The explanatory factors, efficiency, influencing both borders of a partially different On the one hand, the skills of employees are associated with a positive efficiency regardless of the size of the firm and the sector. On the other hand, the age of the company has a positive / negative or no significant both epm and eg based on the activity sector influence. Capital intensity generally is inversely related to efficiency in the epm or no significant mainly in eg. Finally, efficiency levels and patterns of behavior in the period of analysis presented heterogeneity within and among sectors.