Add support for XML_PARSE_HUGE.

You would add it in FeedsXPathParserXML.inc in this line:
$success = $doc->loadXML($raw);

Changing this to a feature request as someone else might want to implement this.

I created the requested patch but am not able to upload the patch on Drupal.org

Therefore I supply the raw patch code here

diff --git a/FeedsXPathParserXML.inc b/FeedsXPathParserXML.inc
index 85eece8..9b8e850 100644
--- a/FeedsXPathParserXML.inc
+++ b/FeedsXPathParserXML.inc
@@ -26,7 +26,15 @@ class FeedsXPathParserXML extends FeedsXPathParserBase {
     }
     $doc = new DOMDocument();
     $use = $this->errorStart();
-    $success = $doc->loadXML($raw);
+    // Check if filesize is larger than 10MB and PHP version is higher than 5.2.
+    // If these criteria are met the constant LIBXML_PARSEHUGE needs to be
+    // passed.
+    if (drupal_strlen($raw) >= 1048576 && version_compare(phpversion(), '5.2', '>')) {
+      $success = $doc->loadXML($raw, LIBXML_PARSEHUGE);
+    }
+    else {
+      $success = $doc->loadXML($raw);
+    }
     unset($raw);
     $this->errorStop($use, $source_config['exp']['errors']);
     if (!$success) {

I just noticed the "Upload new files" link which I read as a part of the issue's body text.
Here is the uploaded patch.
I also committed it to git.

strlen() isn't the correct check here. LIBXML_PARSEHUGE raises the limits on the size of text nodes, as well as, the depth of the document.

We should probably just add this option if it is supported. Are there drawbacks?

Check needs to take into account: Only available in Libxml >= 2.7.0 (as of PHP >= 5.3.2 and PHP >= 5.2.12)