The size of organelles and cellular structures needs to be tightly regulated and coordinated with overall cell size. A well-studied example is the Cdc42-driven polarization and subsequent septin ring formation in Saccharomyces cerevisiae, where the size of the resulting structures scales with cell size. However, the mechanisms underlying this scaling remain unclear. Here, we combine live-cell imaging, genetic perturbations, and three-dimensional mathematical modeling to investigate how septin ring size is controlled. Our integrative approach reveals that positive feedback in the polarization pathway, together with an increase of the amount of polarity proteins as cell size grows, can explain the scaling of the Cdc42 cluster and, consequently, septin ring diameter. Additionally, we show that in cells lacking the formin Bni1, where F-actin-cable assembly and directed polarization are disrupted, exocytosis becomes diffuse, leading to abnormally large septin rings. By integrating new experimental findings and mathematical modeling of yeast polarization, our study provides insights into the origin of septin ring size control.