OK, It took me about 6 hours but I found the answer 
The problem is related to the DPI Scaling. I have a 4k monitor and the DPI Scaling is set to 175% (1.75) and UE was using that to scale the text and borders (anything slate related really) so everything was being scaled by 75% more.
Solution? Go to Project Settings → User Interface → and check “Allow Hight DPI in Game Mode”.
That’s it. Now when I package I get the exact same quality I get in the editor