The key is using H.265 codec (HEVC) instead of H.264, which can cut file sizes roughly in half while keeping similar quality - most video editing software like DaVinci Resolve or Premiere Pro support this, and it's what YouTube prefers anyway. If you want something simpler, tools like HandBrake are free and let you dial in exactly how much compression you want by adjusting the bitrate (I'd suggest starting around 15-20 Mbps for 4K and seeing how it looks). Also consider dropping the frame rate from 60fps to 24fps if your content doesn't need the smoothness, since that alone cuts data significantly. The trick is exporting at a bitrate that's high enough your phone footage still looks crisp but low enough to actually save space - takes some trial and error but beats re-shooting everything.