diff --git a/lib/kolab_sync_backend_state.php b/lib/kolab_sync_backend_state.php
--- a/lib/kolab_sync_backend_state.php
+++ b/lib/kolab_sync_backend_state.php
@@ -72,7 +72,7 @@
         $data = parent::object_to_array($object);
 
         if (is_array($object->pendingdata)) {
-            $data['pendingdata'] = json_encode($object->pendingdata);
+            $data['pendingdata'] = gzdeflate(json_encode($object->pendingdata));
         }
 
         return $data;
@@ -86,7 +86,10 @@
         $object = parent::get_object($data);
 
         if ($object->pendingdata) {
-            $object->pendingdata = json_decode($object->pendingdata, true);
+            $inflated = gzinflate($object->pendingdata);
+            // Inflation may fail for backward compatiblity
+            $data = $inflated ? $inflated : $object->pendingdata;
+            $object->pendingdata = json_decode($data, true);
         }
 
         return $object;