Focused ion beam (FIB) technology has been demonstrated to be a powerful tool in microfabrication, primarily for semiconductor related processing. The migration of the critical dimensions of devices toward the nanometer regime necessitates the knowledge of fundamental milling process mechanisms to gain full use of the opportunities offered by the focused ion beam technique. We have investigated the influence of declining feature sizes on the focused ion beam response of GaAs. Based on our experimental results, a sputter yield promoting the self-focusing effect combined with a sputter rate increase at oblique angles, an opposing dose deficiency effect, and material redeposition for milling aspect ratios > 1 are identified to be responsible for the complex sputter response of GaAs. In addition, the observed preferential etching of arsenic results in local precipitates of mobile Ga-rich residues influencing the fundamental characteristics of FIB patterning, like sputter yield, crater bottom flatness, and ripple formation.