If it’s a gtk app and you are scaling plasma based on qt, this can normally be fixed in your .profile by appending some options.
See point 3 in my ten tweaks Ten tweaks for EOS i3 wm on lenovo with high dpi
In that case you would only add entries for gtk and scale to your liking (down or up). You will need to logout and back in for changes to take effect. Example:
export GDK_SCALE=2
export GDK_DPI_SCALE=0.5
I have the same problem on xfce (gtk) when I run qtcreator based and have to scale qt based apps.
Good luck!