This paper investigates the relationship of enterprise size to the constraints and objectives of Enterprise Resource Planning (ERP) systems adoption. The survey data based on the responses of 44 companies indicates that significant differences exist between small, medium-sized and large enterprises in ERP system adoption. Specifically, the findings suggest that small companies experience more knowledge constraints than their larger counterparts in ERP adoption. Further, while being the most prevalent objective for ERP adoption in all the company groups, business development through ERP adoption is considered especially important by the medium-sized and large enterprises. Finally, the findings of the study suggest that instead of considering small and medium-sized enterprises as one homogenous group, the differences between these two groups of companies should be acknowledged in the future research.