I wonder if you need to add a viewport meta tag to your HTML to help define the initial size that your site should show up on mobile devices, here's a common setting many sites use:
<meta name=viewport content="width=device-width, initial-scale=1">
You can read more about the viewport meta tag in HTML here:
Also, sometimes there can be a difference between how wide a website is in portrait and landscape modes on the same device. Some mobile devices like to zoom in a little bit when you rotate so that it shows the same amount of web page as it did before you rotated, if you want to stop that from happening you can add this line to your CSS:
html { -webkit-text-size-adjust: 100%; }
You can read more about text-size-adjust in CSS here:
That will make sure that the website is the same zoom level whether it's in portrait or landscape mode.
Hope this helps!