urxvt not picking up the correct DPI

I'm running Manjaro Gnome on a HiDPI display. Most things work fine but some programs like urxvt or rofi don't seem to pick up the correct system-wide DPI settings and look very TINY. I thought I could add something Xft.dpi to my .XResources but it doesn't have any effect. I know it's picking up the configs in that file because I can change colours etc and they're applied to urxvt but not the DPI for some reason. Should I poke something else as well?