These files typically use high-efficiency codecs like H.264 or H.265 (HEVC) . By optimizing the bitrate—the amount of data processed per second—encoders can reduce a standard 1.2GB file down to roughly 300MB with minimal visible artifacts.
"Extra quality" in this context refers to the use of advanced video codecs, most commonly HEVC (High Efficiency Video Coding) . These codecs allow for: Higher Data Efficiency 300mb movies 9x press extra quality