Where video size actually comes from
Resolution is the biggest lever: 4K phone footage carries nine times the pixels of 720p, and for a video watched in a chat window that difference is invisible. The quality preset is the second lever — Economy re-encodes at a compression level that looks fine on a phone screen. Length is the third: cutting dead seconds with Trim Video first is free, lossless, and often saves more than compression itself.
The compression runs on FFmpeg in your browser — the video never leaves your device, which matters precisely for the personal videos people most often need to shrink. The before→after readout appears on the file row, and the result plays in the page so you can judge the quality before downloading.