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.