One possible solution is to switch your laptop resolution to 1920x1080 (I know.. it's not native). Then make sure you set all HiDPI settings back to use 1920x1080. Now if you connect your external display at 1920x1080 it will show everything at "normal" size.
Depending on how advanced your windows manager is you can also do it the other way around by upscaling external monitor, like this: https://askubuntu.com/questions/393400/is-it-possible-to-have-two-different-dpi-configurations-for-two-different-screen
assuming you run Gnome by default for Ubuntu 15.04.