Run custom loop for taxonomy archive for proper formatting, use pre_get_posts to change query to show all posts-per-page

functions.php

/**
 * Show header on archive pages
 */
function mb_archive_header() {
  global $post;

	echo '<h1 class="archive-title">Archive for &#8216;' . single_cat_title('', false) . '&#8217;</h1>';
}

/**
 * Show all posts on archives instead of 8 per page
 */
function mb_archive_show_all( $query ) {
  $query->set( 'posts_per_page', -1 );
	return;
}

/**
 * Show Portfolio list on taxonomy archive page
 */
function mb_portfolio_taxonomy_page_loop() {
	global $post;

	if ( have_posts() ) :
		echo '<div class="workthumbs clearfix">';
		while( have_posts() ): the_post();
			mb_display_portfolio();
		endwhile;
		echo '</div><!-- .workthumbs -->';
		do_action( 'genesis_after_endwhile' );
	endif;
}

taxonomy-portfolio_categories.php

<?php

/** Force full width layout */
add_filter( 'genesis_pre_get_option_site_layout', '__genesis_return_full_width_content' );

/** Show archive header */
add_action('genesis_before_loop', 'mb_archive_header');

/** Show all posts on archives instead of 8 per page */
add_action( 'pre_get_posts', 'mb_archive_show_all' );

/** Use our own loop to display content properly */
remove_action( 'genesis_loop', 'genesis_do_loop' );
add_action( 'genesis_loop', 'mb_portfolio_taxonomy_page_loop' );

genesis();