The knowledge of the granular segregation phenomenon induced by size or density differences in the ubiquitous polydisperse systems is important, hence numerous studies have focused on this for the simple yet practical rotating drum. However, in view of the distinctly different characteristics of the two regions (namely, active and passive) for a drum operated in the rolling regime, an understanding of the segregation dynamics in each region is warranted, but remains a gap to date. Accordingly, this study aimed at numerically studying the segregation dynamics of the solid phase in a three-dimensional rotating drum consisting of a binary-size mixture via the Discrete Element Method (DEM). The results demonstrate that the total kinetic energy, the angle of repose, the time-averaged streamwise (i.e., parallel to the bed surface) velocity and the position of the active-passive interface are global parameters, which are not influenced by size-segregation and thereby provides a critical basis for comparing different systems of various polydispersities. Also, the quick onset of the rapid radial segregation leads to sharp initial changes of the variables associated with the two particle types, after which the slow axial segregation leads to a gradual change of these variables over time. Furthermore, size-segregation leads to the redistribution of the particle number of the two particle types in the active and passive regions. Relative to the monodisperse system, the collision forces exerted on the small and large particles are slightly higher and lower, respectively. The results here provide important insights on the dynamics associated with the inevitable segregation phenomenon, which contributes to better operation and predictive capability of the rotating drum. (C) 2017 Elsevier Ltd. All rights reserved.