The nutritional composition of insect-derived meals is strongly influenced by insect diet, while defatting can further modulate nutritional quality. However, some defatting methods, such as supercritical CO₂ extraction, depend on sample properties, including density and macromolecule distribution. Therefore, diet-induced changes may affect lipid extraction efficiency and kinetics, a relationship that remains unexplored. This study evaluated the impact of feeding Hermetia illucens larvae with by-products from olive oil industry (olive leaves, OL, at 15, 30 or 50%; dry full-fat olive pomace, OP, at 30, 50, 70, 90%) or quinoa processing (husk, QH, at 15, 30 or 50%) on supercritical CO₂ defatting performance, meal composition, amino acid profile and digestibility. Despite diet-induced variations in lipid accumulation, defatted kinetics mainly depended on the content and solubility of extractable material, while differences in packed bed microstructure had a minor effect. Protein-rich meals were obtained (25–35%), although most diets reduced protein content, except OP50. QH15 and OP30 worsened essential amino acids in meals, whereas OP50 improved them. Chitin content increased, especially for OP-based meals. Digestibility slightly improved with OP30, OP70, QH15, and QH50. These results show the potential of olive oil and quinoa by-products to be up-cycled by H. illucens into high-value insect meals, without compromising the processing by supercritical CO₂ defatting, supporting sustainable insect-based food and feed production.