[FFmpeg-user] -metadata changes file size: WHY?

[Seattle Chaz]

Greetings all:

Question about unexpected outcome of inserting a metadata title into a
video.  

Start with 3k .bmp images - render to video.mp4 : size = 90+ mb
overlaid video.mp4  with logo: size = 90+ mb
Add video.mp4 audio and a title image via openshot: size = 90+ mb

insert -metadata title into video.mp4 : size = 17 mb???

So far as I can tell, there has been no audio or video degradation.
What the h - e - double hockey sticks is happening here?