Deletes Wordpress's resized images copies. Make site backup, put file in web root catalog and execute. Require PHP >= 5.4.

deleteThumbs.php

<?php
function formatBytes($bytes)
{
    if ($bytes >= 1073741824)
        $bytes = number_format($bytes / 1073741824, 2) . ' GB';
    elseif ($bytes >= 1048576)
        $bytes = number_format($bytes / 1048576, 2) . ' MB';
    elseif ($bytes >= 1024)
        $bytes = number_format($bytes / 1024, 2) . ' KB';
    elseif ($bytes > 1)
        $bytes = $bytes . ' bytes';
    elseif ($bytes == 1)
        $bytes = $bytes . ' byte';
    else
        $bytes = '0 bytes';

    return $bytes;
}

require_once __DIR__ . '/wp-blog-header.php';

global $wpdb;
$uploadDir = wp_upload_dir();

$table = $wpdb->postmeta;
$rows = $wpdb->get_results("SELECT * FROM $table WHERE meta_key = '_wp_attachment_metadata'");

$freedSpace = 0;

foreach ($rows as $row) {
    $temp = unserialize($row->meta_value);
    $fileDir = $uploadDir['basedir'] . '/' . dirname($temp['file']);
    if (isset($temp['sizes'])) {
        foreach ($temp['sizes'] as $size => $info) {
            $filePath = $fileDir . '/' . $info['file'];
            $freedSpace += filesize($filePath);
            echo 'deleted: ' . $filePath . PHP_EOL;
            unlink($filePath);
        }
        unset($temp['sizes']);
        $data = ['meta_value' => serialize($temp)];
        $wpdb->update($table, $data, ['meta_id' => $row->meta_id]);
    }
}

echo 'Total freed space: ' . formatBytes($freedSpace) . PHP_EOL;