Inspired by the other answer - namely this: https://mpv.io/manual/master/#options-sub-scale, where what it says is:
--sub-scale=<0-100> Factor for the text subtitle font size (default: 1).
This affects ASS subtitles as well, and may lead to incorrect subtitle
rendering. Use with care, or use --sub-font-size instead.
Therefore, looking up
--sub-font-size, we see this:
--sub-font-size= Specify the sub font size.
The unit is the size in scaled pixels at a window height of 720. The
actual pixel size is scaled with the window height: if the window
height is larger or smaller than 720, the actual size of the text
increases or decreases as well. Default: 55.
So - create the file
config in a folder
~/.mpv with a line like
to get 80% of the 55 default size.
Log out and back to see changes.