A lot depends on a lot. Like if your project settings match your video specs. Does it?
So "match source" does not actually match the source, I have to change the project settings manually to whatever my video source is?
Match Source matches many of the specs of the original footage (resolution, frame rate, bit rate) -- but it doesn't produce an exact clone of the original footage. Especially when you're working with a format, like MP4, whose file size can vary widely depending on compression, iFrames, etc.
Filesize is determined by duration x bitrate.
I would not use match source but use a preset.
If the file is for Youtube, web, pc set fields to progressive.