The converter reads the GIF's frame delays instead of forcing every frame to the same speed. A browser version of FFmpeg loads only when you start conversion, decodes each animation frame, composites transparent pixels over your chosen color, and encodes a silent H.264 or VP8 video.
Video codecs can reuse visual information between frames, so animated GIFs with large or photographic frames often become smaller as MP4 or WebM. Very tiny GIFs can be an exception because a video container and its keyframes add overhead.