Why did Gustav Mahler use very large orchestras in some of his symphonies?

Gustav Mahler utilized large orchestras in his symphonies primarily because he was a brilliant and fastidious orchestrator who possessed an expert understanding of the capabilities of various instruments. As a professional conductor, he knew how to leverage the variety and number of instruments to create a "marvelously varied tapestry of sound" and achieve richer harmonies by layering different instrumental voices.12

His use of large forces was driven by specific artistic goals:

• Tonal Color and Texture: The vast array of instruments provided Mahler with a wealth of tonal colors, allowing him to craft complex, nuanced soundscapes.23
• Grandeur and Expression: In works like his Symphony No. 8, the massive scale—including choirs, vocal soloists, and an expanded orchestra—was intended to reflect the "whole universe" bursting into song, serving as a monumental statement of human and religious aspirations.43
• Structural Clarity: Despite the large resources, Mahler handled them with restraint and sensitivity, ensuring that his symphonies remained logically structured beneath the wealth of detail.1

Ultimately, Mahler’s orchestration was a deliberate tool to realize his specific aural visions and emotional intentions.3