If you view source in the HTML that Asciidoctor generates
You'll find in the HEAD
<style>
/* Asciidoctor default stylesheet...
...
#header,#content,#footnotes,#footer{width:100%;margin-left:auto;margin-right:auto;margin-top:0;margin-bottom:0;max-width:62.5em;*zoom:1;position:relative;padding-left:.9375em;padding-right:.9375em}
Comment out this
/*max-width:62.5em;*/
That will remove the restriction and make your content full width