MKVToolnix bulking up files

[XenoPhex]

I recently tried to create an MKV file with the following streams:
Video 1.MP4 - 407.4MB (H.264 encoded file, no audio)
Sub 1.srt - 64KB
Audio 1.MP3 - 55.3MB (LAME encoded file)
Audio 2.MP3 - 55.3MB (LAME encoded file)

When I mux the file with MKVToolnix (GUI), I seem to get a file with a size of 703.5MB and not around 520MB (the added totals of all the streams). I understand if it's a little higher then 520MB for the MKV container info, but an extra 200MB seems absurd. Does anyone have an explanation for the 200MB of bulk?

[Baldrick]

Please don't cross post. Continue in https://forum.videohelp.com/topic364944.html .