Font size too large in Sylpheed, Okular, etc

Some programs have very large fonts, e.g. Okular, Sylpheed, as well as the notifications (dunst). I’ve tried changing the font size using “Customize look and feel” but it doesn’t change anything.

I’m using a nvidia GPU. I used to use Endeavour without the GPU enabled in the bios and didn’t have this problem then.

Any ideas?

Cheers