OSC font size too small on KDE Plasma. How do I change it?

[o-alquimista]

The default OSC font is very small on Plasma, and it's even smaller for portrait-oriented videos.

Out of the box, it's much better on GNOME and other GTK desktops. (will take some screenshots later to compare)

Currently, this is what I have at ~/.config/mpv/script-opts/osc.conf:

layout=slimbox
timems=yes

mpv 0.40.0 from Fedora (43) packaging.

I've been trying different combinations of scalewindowed and vidscale=no, but these options don't seem to affect the font size in any way. What can I do?