I don't think this is PDO specific. I believe this is just how mysqlnd allocates memory. It would be worth looking into it in more detail to see if this could be improved.

<?php
ini_set('memory_limit', -1);
$mb = 1024**2;
$n = 100 * $mb;

$m = memory_get_usage();
$x = str_repeat('a', $n);
printf("%.2F\n", (memory_get_usage() - $m) / $mb);

$db = new pdo('mysql:host=127.0.0.1', 'root', '', [
    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION
]);
$m = memory_get_usage();
$q = $db->query("select repeat('a', $n)");
printf("%.2F\n", (memory_get_usage() - $m) / $mb);
$a = $q->fetchAll();
printf("%.2F\n", (memory_get_usage() - $m) / $mb);

PHP 8.0:

100.00
636.05
736.06

PHP 8.1:

100.00
536.05
636.05

Using PDO::MYSQL_ATTR_USE_BUFFERED_QUERY = false:

100.00
0.02
100.02

I removed an unnecessary 100 MB copy in PHP 8.1, but the memory usage is still larger than it should be. Of course, when working with large data using an unbuffered query is preferred, which removes the memory overhead entirely.

The problem is that we receive the packet in 16MB chunks (with the total size unknown) and continue reallocate the buffer for each chunk (https://github.com/php/php-src/blob/7f8ea83ef438fbcfa1cbc636d701491d4e773245/ext/mysqlnd/mysqlnd_wireprotocol.c#L1409). Because this uses an arena allocator, "realloc" effectively becomes "alloc" and memory usage will grow quadratically with the payload size.