Why compressed blob values don't change sqlite database size?

Question

I have a .sqlite database:

$db = new PDO("sqlite:db.sqlite");

It's size is 60MB. It has 1 table. One of this table columns (the only heavy one) holds BLOB_TEXT data. I am trying to compress this column values:

$pdo = $db->query("SELECT id, house_plan FROM houses");
$houses = $pdo->fetchAll(PDO::FETCH_ASSOC);
$initial_length = 0;
$compressed_length= 0;

for ($i = 0; $i < count($houses); $i++) {
  $id = $houses[$i]["id"];
  $house_plan = $houses[$i]["house_plan"];
  $compressed = gzcompress($house_plan);

  $initial_length += strlen($house_plan);
  $compressed_length += strlen($compressed);
  flush();

  $query = $db->prepare("UPDATE houses SET house_plan=? WHERE id=" . $id);
  $query->bindParam(1, $compressed, PDO::PARAM_LOB);
  $query->execute();
}

Then printing summed initial BLOB_TEXT data length and compressed data length (which replaced the initial data in the table):

echo "Before: " . $initial_length . ", After: " . $compressed_length;
// Before: 52222553, After: 2577948

And their difference in %:

echo "Difference: " . (100 - round($compressed_length / $initial_length * 100)) . "%";
// Difference: 95%

Afterwards I check the size of the database, and it's still 60MB. BLOB_TEXT data difference was 95%, but the size of the database is not changed.

How that might be? Or what I am doing wrong compressing BLOB_TEXT?
I expect database size reduces as well, because this is the only heavy column I have.

Answer 1

Before the compression, the database was filled to about 100 % with data. After the compression, it contains about 5 % data and about 95 % free space.

Usually, it is a good idea to avoid reducing the file size because the free space will be filled up with new data later anyway. If you want to re-pack the database, run VACUUM.