It has nothing to do with the viewport meta tag. If you open the page on the desktop and slowly close the window you will see that a horizontal scrollbar appears at around 790px width.
It is this overflowing content that squashes the page on smaller devices as they need to fit all 790px into their smaller screens which they can only do by shrinking the content accordingly.
You must avoid horizontal scrollbars on the viewport at all times where mobile is concerned.
The problem element is your popover element which is lying at the side of the page making it too wide for mobile. Just temporarily make it display:none for now and you will see that it is indeed the element causing the problem.
You need to move it away from the edge of the right side of the viewport at smaller widths and make sure it doesn’t stick out.