the post thumbnail won't apply thumbnail sizes set in media

I’m a little confused, I want to show in my homepage a gallery of 4 proyect images taken from a custome post type called portfolio, this is the code to call the images:

<?php $args = array( 'post_type' => 'portfolio', 'posts_per_page' => 4 );?>
				 <?php $loop = new WP_Query( $args );?>
				 <?php while ( $loop->have_posts() ) : $loop->the_post(); ?>
				<a>/portfolio/"><?php the_post_thumbnail('thumbnail',array( 'class' => 'frame')); ?></a>
				 <?php endwhile;	?>

They show Ok, but I’m having trouble getting the size I set in the dashboad>settings>media
there I changed the thumbnail size to 160 x 100 and Crop thumbnail to exact dimensions (normally thumbnails are proportional) is checked, The wierd thing is that I get 100×100 px images instead, not sure why, anyhelp is appreciated.