Try it
<div class="main-interior portfolio" id="portfolio-big-pics" style="display: block;">
<?php
$args = array( 'post_type' => 'portfolio', 'order' => 'ASC');
$loop = new WP_Query( $args );
$img_titles="";
while ( $loop->have_posts() ) : $loop->the_post();
$extraLastClass = $loop->current_post + 1 === $loop->post_count ? ' main-image-porfolio-main' : '';
the_post_thumbnail( "thumbnail", array( "class" => "main-image portfolio $extraLastClass" ) );
$img_titles .= '<div class="portfolio-box">
<h5>Portfolio</h5>
<ul class="item-list" id="portfolio-list">
<li><a href="'.get_permalink().'">'.get_the_title().'</a>
</li>
</ul>
</div>';
endwhile;
?>
</div>
<?php echo $img_titles; ?>