The study compares two measures of organizational size of Higher Education Institutions widely used in the literature: the number of Academic Personnel (AP) measured according to definitions from international education statistics, and the Scientific Talent Pool (STP), i.e. the number of unique authors affiliated to the HEI as derived from the Scopus database. Based on their respective definitions and operationalizations, the researchers derive expectations on the factors generating differences between these two measures , as related to the HEI’s research orientation and subject mix, as well as to the presence of a university hospital. The paper tests these expectations on a sample of more than 1,500 HEIs in Europe by combining data from the European Tertiary Education Register and from the Scimago Institutions Ranking. The results provide strong support to the expected relationships and also highlights cases where the institutional perimeter of HEIs is systematically different between the two sources. The researchers concludes that these two indicators provide complementary measures of institutional size, one more focused on the organizational perimeter as defined by employment relationships, the other on the persons who contribute to the scientific visibility of HEIs. Comparing the two indicators therefore is likely to provide a more in-depth understanding of the HEI resources available. Wolfgang Glänzel, KU Leuven has been involved as a discussant.