The font gets extracted from the screen itself by default to compensate low/high resolution situations. This prevents to small or too big bubble sizes due to the used font size.

The last resort is "Lux/18", if for some reason no other font got found.

So if the font in general is to small, check the font used for the screen in MUI settings. If the field is blank, MUI is using the global systems screen font and size.

Also keep in mind that a custom non MUI public screen may be fully defined by the opening application. It may use the system font from system settings as well unless the application which opened the screen has own font requirements and settings.

In general the bubble font can be overridden by using the per notification configuration like roschmyr wrote, but this will be alot of work and should not be required.