How to make a video smaller
Drop in a video. On this page the quality starts at 25 rather than 55, and a video bigger than 720p starts at 720p. The line under the quality slider gives the new frame size, the length, and about how big the file will be; it changes as you change anything. Then press Save video.
What makes the biggest difference
Size sets how many pixels each frame has: 720p has under half the pixels of 1080p, and 480p about a fifth. Quality sets how many bits each pixel gets. Length counts in full, so trimming off what you don't need is the easiest saving, and leaving the sound out saves about 1 MB a minute.
How close the estimate is
The estimate is the bitrate times the length. The encoder aims for that bitrate but spends more on busy, moving scenes and less on still ones, so the finished file can land somewhat above or below it. The done card gives its real size.
Questions
How do I get a video under 25 MB?
25 MB is 200 megabits, so a 60-second clip can use about 3.3 Mbps in all. Lower Size and Quality until the estimate is a few MB under the limit, to leave room, then check the size on the finished file.
MP4 or WebM?
MP4 (H.264) plays on every phone and in every app. WebM (VP9) is often smaller at the same quality, but some apps and older iPhones won't play it. If your browser can't write H.264, only WebM is offered.
Can a video get smaller without being encoded again?
Not by much. Making the picture smaller means encoding it again; this page does it with your browser's own video encoder, so the file never leaves your device.