In many WordPress templates menu is not extended to the entire width of the element in which it is located. Scalable trying to make the menu that fits the width of the div in which it is located.
My header.php
<div class="navigation-menu">
<div id="navbar" class="header-bottom navbar default">
<nav id="site-navigation" class="navigation main-navigation" role="navigation">
<h3 class="menu-toggle"><?php _e( 'Menu', 'templatemela' ); ?></h3>
<a class="screen-reader-text skip-link" href="#content" title="<?php esc_attr_e( 'Skip to content', 'templatemela' ); ?>"><?php _e( 'Skip to content', 'templatemela' ); ?></a>
<div class="mega-menu">
<?php wp_nav_menu( array( 'theme_location' => 'primary','menu_class' => 'mega' ) ); ?>
</div>
</nav><!-- #site-navigation -->
</div>