Try removing the <div class="xzoom-thumbs"> from the loop.
<div class="col-left">
<div class="xzoom-thumbs">
<img class="xzoom" src="<?php the_field('preview_image'); ?>" xoriginal="<?php echo the_field('preview_image'); ?>" />
<?php
if( have_rows('main_gallery') ):
while ( have_rows('main_gallery') ) : the_row();
<a href="<?php the_sub_field('image_preview') ?>">
<img class="xzoom-gallery" width="80" src="<?php the_sub_field('image_thumb'); ?>" xpreview="<?php the_sub_field('image_preview'); ?>">
</a>
<?php
endwhile;
endif;
?>
</div>
</div>
