Hi there,
that Site imports some Customizer > Additional CSS to constrain the content width:
.no-sidebar:not(.generate-columns-activated) .inside-article > *, .no-sidebar #comments {
max-width: 700px;
margin-left: auto;
margin-right: auto;
}
If you want to remove that styling from the pages then replace it with this CSS:
.no-sidebar:not(.generate-columns-activated):not(.page) .inside-article > *, .no-sidebar #comments {
max-width: 700px;
margin-left: auto;
margin-right: auto;
}