GIFs autoplay everywhere (Slack, Discord, README files, forums) without needing a video player, which is why short MP4 clips so often get turned into them. GIF's color palette is limited to 256 colors and carries no audio track, so very colorful or long clips can come out large and slightly banded. Trimming the clip to a few seconds first with DuckConvert's Trim Video tool keeps the output snappy.
256 colours, and what that does to your video
GIF predates the web's ability to show real video and its palette reflects that: each frame can use at most 256 distinct colours. Video routinely contains tens of thousands. The conversion therefore has to choose a palette and then approximate everything else, usually by dithering, scattering pixels of available colours so that from a distance they read as the colour that is missing.
Smooth gradients are where this shows. A sky, a soft shadow or a slow fade turns into visible banding or a faint speckled texture. Screen recordings, line art and anything with flat blocks of colour survive almost untouched, which is why UI demos make far better GIFs than camera footage.
Why the GIF is bigger than the video was
This surprises everyone. GIF compresses each frame with an algorithm designed for small graphics in 1987, with none of the motion prediction that lets modern video describe a frame purely in terms of how it differs from the one before. A ten-second clip that occupied two megabytes as an MP4 can easily become fifteen megabytes as a GIF.
The levers that actually help are duration and dimensions: halve the width and you roughly quarter the size. Keep clips to a few seconds. If the destination accepts a video file at all, an MP4 or WebM will look better and weigh less, and most places that historically wanted a GIF now accept both.