Labour productivity differs significantly between small, medium-sized and large enterprises in the private enterprise sector. This holds both for the level of labour productivity as well as its development. Size-class differences regarding the level of labour productivity are only to a limited extent dependent on differences in sectoral structure. A model of the production structure has been developed and estimated to shed some light on these phenomena explaining various aspects of labour productivity of (average) small, medium-sized and large enterprises and applied to four economic sectors in Dutch private enterprise (in a separate paper (Kwaak, (2012), the model is tested for private enterprise as a hole). It should be stressed that the analysis refers to the 'representative' or average enterprise in the small, medium-sized or large enterprise segment of the economy; it does not refer to individual enterprises.