That’s just the default behaviour of VLC to respect the formatting within the subtitles. You can easily tell it to ignore the formatting, and use relative sizes (default setting for “Relative font size” is “Auto”, aka respect formatting within subtitles, and if not present, apply some reasonable sizing).
Example using the same video: https://i.imgur.com/Sey6jRw.png
Same should be possible in Plex; you already have a font size setting for the subtitles, and the default behaviour should be to ignore formatting regarding text size within subtitle files, and rather use the setting in Plex (or at least make it an option to do so).
