We report on a polarization-maintaining (PM) very large mode area (VLMA) Er-doped fiber with a 40 µm hexagonal core, fabricated via the REPUSIL powder process. The fiber’s performance was evaluated in a MOPA configuration under 976 nm cladding-pumping and 1480 nm core-pumping. A numerical model, distinguishing between isolated ions and clusters to account for cooperative up-conversion and pair-induced quenching, shows excellent agreement with experimental results. This validated model provides a robust tool for designing high-energy, eye-safe PM-VLMA fiber amplifiers.