Window controls tiny on Ubuntu

On Ubuntu the window controls (minimise, maximise etc.) are really tiny compared to other applications.

It's not a problem when maximised, as Unity's top bar provides the buttons, but when not maximised the window has to draw its own controls.

The first image below shows Edge's controls. The second shows Chrome's. The third show's gedit (a GNOME app). I would expect Edge's controls to be the same size as the others. 

Edge's top bar is slightly smaller than gedit's, so it may be that the buttons are being scaled to fit.