diff --git a/apps/encryption/tests/CopySizeTest.php b/apps/encryption/tests/CopySizeTest.php
new file mode 100644
index 0000000000000..b04aecbd6f449
--- /dev/null
+++ b/apps/encryption/tests/CopySizeTest.php
@@ -0,0 +1,69 @@
+<?php
+
+declare(strict_types=1);
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\encryption\tests;
+
+use OC\Files\Storage\Temporary;
+use OC\Files\View;
+use OCA\Encryption\KeyManager;
+use OCP\Server;
+use Test\TestCase;
+use Test\Traits\EncryptionTrait;
+use Test\Traits\MountProviderTrait;
+use Test\Traits\UserTrait;
+
+/**
+ * The size of an encrypted file is kept in `unencrypted_size`, which every reader
+ * prefers over `size`. A copy has to carry it over, otherwise the copy is reported
+ * as empty.
+ */
+#[\PHPUnit\Framework\Attributes\Group(name: 'DB')]
+class CopySizeTest extends TestCase {
+	use MountProviderTrait;
+	use EncryptionTrait;
+	use UserTrait;
+
+	private function setUpView(): View {
+		Server::get(KeyManager::class)->validateMasterKey();
+		Server::get(KeyManager::class)->validateShareKey();
+		$this->createUser('test1', 'test2');
+		$this->setupForUser('test1', 'test2');
+		$this->registerMount('test1', new Temporary(), '/test1/files/other');
+		$this->loginWithEncryption('test1');
+
+		return new View('/test1/files');
+	}
+
+	public function testCopyKeepsTheSize(): void {
+		$view = $this->setUpView();
+
+		$view->file_put_contents('source.bin', str_repeat('a', 20000));
+		$this->assertTrue($view->copy('source.bin', 'target.bin'));
+
+		$this->assertEquals(20000, $view->getFileInfo('target.bin')->getSize());
+	}
+
+	public function testCopyToAnotherStorageKeepsTheSize(): void {
+		$view = $this->setUpView();
+
+		$view->file_put_contents('source.bin', str_repeat('a', 20000));
+		$this->assertTrue($view->copy('source.bin', 'other/target.bin'));
+
+		$this->assertEquals(20000, $view->getFileInfo('other/target.bin')->getSize());
+	}
+
+	public function testCopyOfAFolderKeepsTheSizes(): void {
+		$view = $this->setUpView();
+
+		$view->mkdir('source');
+		$view->file_put_contents('source/file.bin', str_repeat('a', 20000));
+		$this->assertTrue($view->copy('source', 'target'));
+
+		$this->assertEquals(20000, $view->getFileInfo('target/file.bin')->getSize());
+	}
+}
diff --git a/build/integration/encryption_features/encryption.feature b/build/integration/encryption_features/encryption.feature
index 15704609d79df..91854555a538f 100644
--- a/build/integration/encryption_features/encryption.feature
+++ b/build/integration/encryption_features/encryption.feature
@@ -26,6 +26,7 @@ Feature: encryption
     And User "user0" adds a file of 20000 bytes to "/big.bin"
     When User "user0" copies file "/big.bin" to "/copy.bin"
     Then the HTTP status code should be "201"
+    And File "/copy.bin" should have prop "d:getcontentlength" equal to "20000"
     When Downloading file "/copy.bin"
     Then the HTTP status code should be "200"
 
diff --git a/lib/private/Files/Cache/Cache.php b/lib/private/Files/Cache/Cache.php
index 7c8023c90c1d6..1e729c33cd1fd 100644
--- a/lib/private/Files/Cache/Cache.php
+++ b/lib/private/Files/Cache/Cache.php
@@ -1316,8 +1316,14 @@ private function cacheEntryToArray(ICacheEntry $entry): array {
 			$data['permissions'] = $entry['scan_permissions'];
 		}
 
-		if ($entry->isEncrypted() && isset($entry['encryptedVersion'])) {
-			$data['encryptedVersion'] = $entry['encryptedVersion'];
+		if ($entry->isEncrypted()) {
+			// the size of an encrypted file is stored in its own column, which every
+			// reader prefers over `size`, so the copy is reported as empty without it
+			$data['unencrypted_size'] = $entry->getUnencryptedSize();
+
+			if (isset($entry['encryptedVersion'])) {
+				$data['encryptedVersion'] = $entry['encryptedVersion'];
+			}
 		}
 
 		return $data;
diff --git a/tests/lib/Files/Cache/CacheTest.php b/tests/lib/Files/Cache/CacheTest.php
index 39e05c590ee98..023e4a76d256f 100644
--- a/tests/lib/Files/Cache/CacheTest.php
+++ b/tests/lib/Files/Cache/CacheTest.php
@@ -602,10 +602,25 @@ public function testCopyFromCachePreservesEncryptedVersion(): void {
 		$this->assertSame(3, $targetEntry['encryptedVersion']);
 	}
 
+	public function testCopyFromCachePreservesUnencryptedSize(): void {
+		$data = [
+			'size' => 128, 'mtime' => 50, 'mimetype' => 'foo/bar',
+			'encrypted' => true, 'encryptedVersion' => 3, 'unencrypted_size' => 100,
+		];
+		$this->cache->put('source', $data);
+		$sourceEntry = $this->cache->get('source');
+		$this->assertEquals(100, $sourceEntry->getUnencryptedSize());
+
+		$this->cache->copyFromCache($this->cache, $sourceEntry, 'target');
+
+		$targetEntry = $this->cache->get('target');
+		$this->assertEquals(100, $targetEntry->getUnencryptedSize());
+	}
+
 	public function testCopyFromCacheClearsEncryptedVersionWhenCopyingToNonEncryptedStorage(): void {
 		$data = [
-			'size' => 100, 'mtime' => 50, 'mimetype' => 'foo/bar',
-			'encrypted' => true, 'encryptedVersion' => 3,
+			'size' => 128, 'mtime' => 50, 'mimetype' => 'foo/bar',
+			'encrypted' => true, 'encryptedVersion' => 3, 'unencrypted_size' => 100,
 		];
 		$this->cache2->put('source', $data);
 		$sourceEntry = $this->cache2->get('source');
@@ -627,6 +642,9 @@ public function testCopyFromCacheClearsEncryptedVersionWhenCopyingToNonEncrypted
 		$targetEntry = $targetCache->get('target');
 		$this->assertFalse($targetEntry->isEncrypted());
 		$this->assertSame(0, $targetEntry['encryptedVersion']);
+		// the target is not marked as encrypted, so its size is read from `size`
+		$this->assertEquals(0, $targetEntry['unencrypted_size']);
+		$this->assertEquals(128, $targetEntry->getSize());
 	}
 
 	public function testGetIncomplete(): void {