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Abstract Optoelectronics are crucial for developing energy‐efficient chip technology, with phase‐change materials (PCMs) emerging as promising candidates for reconfigurable components in photonic integrated circuits, such as nonvolatile phase shifters. Antimony sulfide (Sb₂S₃) stands out due to its low optical loss and considerable phase‐shifting properties, along with the non‐volatility of both phases. This study demonstrates that the crystallization kinetics of Sb₂S₃can be switched from growth‐driven to nucleation‐driven by altering the sample dimension from bulk to film. This tuning of the crystallization process is critical for optical switching applications requiring control over partial crystallization. Calorimetric measurements with heating rates spanning over six orders of magnitude, reveal that, unlike conventional PCMs that crystallize below the glass transition, Sb₂S₃exhibits a measurable glass transition prior to crystallization from the undercooled liquid (UCL) phase. The investigation of isothermal crystallization kinetics provides insights into nucleation rates and crystal growth velocities while confirming the shift to nucleation‐driven behavior at reduced film thicknesses—an essential aspect for effective device engineering. A fundamental difference in chemical bonding mechanisms was identified between Sb₂S₃, which exhibits covalent bonding in both material phases, and other PCMs, such as GeTe and Ge₂Sb₂Te₅, which demonstrate pronounced bonding alterations upon crystallization.