Short answer
Video size depends mostly on duration, resolution, frame rate, codec, and bitrate.
What to check
Review resolution, target bitrate, frame rate, audio, and visual checks. Check video and audio codecs, duration, frame rate, resolution, aspect ratio, subtitles, and audio synchronization after conversion.
Using mp4.now
Use the MP4 TO WEBM converter, keep the source, and inspect the downloaded result before sharing it.