I am using the Stay theme and have set up the website and all looks good until I view it on my tablet then the main image is twice the width of the page so overhangs the rest of the page and doesn’t look great.
The Stay theme is mobile compliant and the demo theme doesn’t do this so why does mine?
Responsive width means the layout adapts depending on the size of the device being used to view your site. When responsive width themes are viewed on mobiles sidebars appear below the posts in order to provide as much space as possible for reading.
If you have enabled the mobile theme here > Appearance > Mobile then disable it.
“the demo theme doesn’t do this so why does mine?”
Because the demo doesn’t have a page like the one you created.
Your page is a “Home” template page, with a featured image set to it. In contrast to the other templates, the featured image in the Home template doesn’t shrink, because it’s missing the max-width:100% rule. This needs to be fixed by WP.