add_action( 'admin_head-media-upload-popup', 'make_media_thumbs_bigger' );

function make_media_thumbs_bigger() 
{
    ?>
    <style>.media-item.preloaded{height:180px !important}</style>
    <script type="text/javascript">
        jQuery(document).ready( function($) {
            $('#media-items img').each(function(){
                $(this).attr('width','150').css('max-width','100%');
                $(this).attr('height','150').css('max-height','100%');
            });

           $('#media-items .media-item.preloaded').each(function(){
                $(this).css('height','380px');
            });
        });     
    </script>
    <?php
}