-        Cursor childrenLengths = query(db, selectUri, selectProjection, selectWhere, whereArgs, null);
-        if (childrenLengths != null && childrenLengths.moveToFirst()) {
-            long size = 0;
-            while (!childrenLengths.isAfterLast()) {
-                size += childrenLengths.getLong(childrenLengths.getColumnIndex(ProviderTableMeta.FILE_CONTENT_LENGTH));
-                childrenLengths.moveToNext();
+        
+        // read current size saved for the folder 
+        long folderSize = 0;
+        long folderParentId = -1;
+        Uri selectFolderUri = Uri.withAppendedPath(ProviderTableMeta.CONTENT_URI_FILE, folderId);
+        String[] folderProjection = new String[] { ProviderTableMeta.FILE_CONTENT_LENGTH,  ProviderTableMeta.FILE_PARENT};
+        String folderWhere = ProviderTableMeta._ID + "=?";
+        Cursor folderCursor = query(db, selectFolderUri, folderProjection, folderWhere, whereArgs, null);
+        if (folderCursor != null && folderCursor.moveToFirst()) {
+            folderSize = folderCursor.getLong(folderCursor.getColumnIndex(ProviderTableMeta.FILE_CONTENT_LENGTH));;
+            folderParentId = folderCursor.getLong(folderCursor.getColumnIndex(ProviderTableMeta.FILE_PARENT));;
+        }
+        folderCursor.close();
+        
+        // read and sum sizes of children
+        long childrenSize = 0;
+        Uri selectChildrenUri = Uri.withAppendedPath(ProviderTableMeta.CONTENT_URI_DIR, folderId);
+        String[] childrenProjection = new String[] { ProviderTableMeta.FILE_CONTENT_LENGTH,  ProviderTableMeta.FILE_PARENT};
+        String childrenWhere = ProviderTableMeta.FILE_PARENT + "=?";
+        Cursor childrenCursor = query(db, selectChildrenUri, childrenProjection, childrenWhere, whereArgs, null);
+        if (childrenCursor != null && childrenCursor.moveToFirst()) {
+            while (!childrenCursor.isAfterLast()) {
+                childrenSize += childrenCursor.getLong(childrenCursor.getColumnIndex(ProviderTableMeta.FILE_CONTENT_LENGTH));
+                childrenCursor.moveToNext();