If no image, bigger text

This should work.

<div class="feature-section-1">
<?php $posts = get_posts( "tag='tagtitle'&numberposts=1" ); ?>
  <?php if( $posts ) : ?>
    <?php foreach( $posts as $post ) : setup_postdata( $post ); ?>
      <a href="https://wordpress.stackexchange.com/questions/210837/<?php echo get_permalink($post->ID); ?>" >
        <?php if ( has_post_thumbnail() ): ?>
          <h3><?php echo $post->post_title; ?></h3>
          <div class="false-excerpt-div"><?php the_excerpt(); ?></div>
          <?php the_post_thumbnail('columner-thumb'); ?>
        <?php else: ?>
          <h1><?php echo $post->post_title; ?></h1>
          <div class="false-excerpt-div"><?php the_excerpt(); ?></div>
        <?php endif; ?>    
      </a>
    <?php endforeach; ?>
  <?php endif; ?>
</div>