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.