Since Gnome 3.14 was released many icons themes have issues with scaling panel icons and system symbols.

Here's an example of a system symbol which is clearly not scaled correctly:

How To Fix it?

I've only managed to test this on "Elementary 3.1" icon theme but with a
little tinkering and understanding I think this will work on most. As long as
the theme uses SVG vector graphics.

Moving on. Choose the theme you would like to edit and go to its directory.
In my case /usr/share/icons/elementary/.

Before proceeding you will need to discover where the defective icons are stored.
In my case that was:

Panel App Draw Icon /usr/share/icons/elementary/actions/symbolic/
System Symbols /usr/share/icons/elementary/status/symbolic/

You will need to remember the last two directorys for reference later (/status/symbolic/ & /actions/symbolic/)

Now edit the file index.theme And find the line [status/symbolic].
Below "Size" add the lines:

MinSize=8
MaxSize=512
Do you the same for the line [actions/symbolic]

Final Result