Thumbnail Sizes

So, I’m using get_the_thumbnail to retrieve the featured image in a post and this is the code I’m using

<?php the_post_thumbnail( array(140,100), array('class' => 'thumbnail')); ?>

the thing is, the size is not coming out at 140×100. Instead it’s at 100×100.

Any ideas what i’m doing wrong?

Heres the full context btw:

<div class="tab-pane active" id="books">
<?php query_posts( array ( 'category_name' => 'new-books', 'posts_per_page' => -5 ) ); ?>

<?php while ( have_posts() ) : the_post();?>
    <ul class="unstyled">
    	<li class="clearfix">
    		<?php the_post_thumbnail( array(100,160), array('class' => 'thumbnail')); ?>
    		&nbsp;
    		<h3 class="pull-right" style="margin:0;"><?php the_content();?></h3>
    	</li>
			<hr>
    </ul>

<?php endwhile; ?>
    <?php wp_reset_query(); ?>
</div>