It should be using the larger version. Make sure you have gnome-icon-theme-full
installed if you're using gnome-shell.
You can change the theme you're using in dconf-editor
by changing the /org/gnome/desktop/interface/icon-theme
entry to be gnome
. It looks like you are currently using the Humanity theme (or ubuntu-mono-dark, which is the Ubuntu default), and it does not provide high resolution versions of that icon. The lack of high resolution versions of the icons in any of the Humanity or ubuntu-mono icon themes may be considered a bug, and you should report it in Launchpad.