Why MP4 files get so large
Phones record MP4 at high bitrates so that nothing is lost while filming: about 16–18 Mbps for 1080p at 30 fps and 40–50 Mbps for 4K, which is 120 MB to 375 MB per minute. That headroom is useful for editing but wasted on a clip you only want to send or keep.
Re-encoding at a bitrate matched to the resolution recovers most of that space. A 1080p30 clip at the Balanced level uses 2.8 Mbps, about 21 MB per minute.
Keeping quality while shrinking
Quality loss shows first in fast motion, fine textures such as grass or water, and dark scenes. If those matter, choose High quality, or lower the resolution one step instead of squeezing the bitrate: 720p at 3 Mbps usually looks cleaner than 1080p at 1.6 Mbps.
The sound is copied unchanged unless you ask otherwise, so voices and music are never touched by this compression.
MP4 in, MP4 out
The output is H.264 video in an MP4 container, the combination every phone, browser, TV and editor plays. If your MP4 holds HEVC (H.265) video, converting to H.264 also fixes playback on Windows PCs without the HEVC extension.
The file name gets a suffix such as -compressed or -720p, so the original is never overwritten.
Reading the result
The result table compares size, resolution, frame rate, bitrate and length of the two files, read from the actual output. If the new file is larger, the original was already efficient; keep it.