Video files are large because video is data-intensive — a single minute of 4K footage at high quality can exceed 500 MB. Compression works by removing information the human eye is unlikely to notice: subtle colour variations, imperceptible motion between frames, and redundant spatial detail.
For a quick compression without fiddling with settings, browser-based tools like fwip handle MP4 compression locally. Drop your file, get a smaller version back. The tool selects reasonable defaults for bitrate and resolution that balance quality against file size.
For more control, HandBrake (free, cross-platform) is the standard tool. Open your MP4, select the "Fast 1080p30" preset for a good starting point, and adjust from there. The key settings that affect file size: resolution (1080p vs 720p), codec (H.265 produces files 30-50% smaller than H.264 at the same quality), and CRF/quality slider (18-23 is the sweet spot for H.264; 24-28 for H.265).
The biggest win is almost always resolution. If your 4K video will be watched on a phone or shared on social media, downscaling to 1080p before compression can reduce the file by 75% with no perceived quality loss at the viewing size.
For command-line users, FFmpeg is the universal tool: `ffmpeg -i input.mp4 -c:v libx265 -crf 26 -preset medium output.mp4` produces a well-compressed H.265 file.