Magnetic skyrmions are typically modeled as rigid particle-like configurations in spintronic applications, yet their thermodynamic dimensions remain highly sensitive to external fields, challenging device reliability. In this work, we demonstrate that individual skyrmions in Pt/Co/Tb multilayers exhibit a robust, invariant diameter across a wide range of temperatures and magnetic fields, even while the macroscopic skyrmion density varies by orders of magnitude. Utilizing a You Only Look Once (YOLO)-based object-detection model for high-throughput statistical analysis of Lorentz transmission electron microscopy (LTEM) images, we map the skyrmion size and density across a comprehensive phase space. This density variation is attributed to the competition among saturation magnetization MS, effective anisotropy Keff, and the Dzyaloshinskii-Moriya interaction D. Crucially, micromagnetic simulations reveal that localized defect-induced energy barriers pin the domain walls, effectively counteracting Zeeman-driven size compression. This defect-mediated mechanism successfully decouples the microscopic skyrmion dimensions from global thermodynamic variables, offering a robust strategy for defect-engineered, size-stable skyrmionic architectures in next-generation high-density memory.