Hi there,
on small devices the content width will be defined by:
The width of viewport minus the left and right padding set in the Customizer > Layout > Container.
So you can reduce the left and right padding for mobile to increase its width.
But you also have this custom CSS on your site:
@media only screen and (max-width: 768px) {
.single-post .inside-article {
padding: 23px !important;
}
}
Which is overriding the customizer padding for single posts.
If you want to keep that, then you can change it like so: padding: 23px 12px !important; – the first value will be top and bottom, and the second value is left and right.