Modeling of particle-size segregation in dense granular flows could benefit from a better understanding of the segregation behavior of individual large grains that are surrounded by smaller grains. In a previous study, the force in the direction of segregation experienced by such a particle was measured and decomposed into a modified Archimedean buoyancy force and a segregation lift force. Here we present a new micromechanical analysis of this granular buoyancy force that connects the microscale contact behavior to the macroscopic force, thereby lending further support to its validity. In the process of this validation we uncover evidence linking the average surface contact density on a grain to the scaling of the buoyancy force with its size. Our findings support the use of the existing Voronoi approximation for calculation of the granular buoyancy force and substantiate the decomposition of the net force into buoyancy and segregation lift. Ultimately, these insights will aid development of new models for size segregation by closely linking micro and macro behavior.