Instancing creation Problem

[Anonymous]

Hiya
So one of our artists didnt like using instances and we came up with a naming convention for the exporter. Howerver the maya file has got way too big (surprise surprise), so I wrote a plugin to got through the file, find all the instances using our naming convention, and turn these things into actual maya instances.
Heres the part where I duplicate the master object instance and replace the instance with it:

                        MFnDagNode instNode(dagPathList); //this is the named instnace

 MObject parent = instNode.parent(0, &stat);
 MFnDagNode parentNode(parent);
 if(stat == MS::kSuccess)
 MGlobal::displayInfo( MString("got parent ") + parentNode.name() );
 else
 continue;

 MFnDagNode originalNode(originalInstPath); //this is the master instance, a dagpath

 MObject duplicate = originalNode.duplicate(true, false, &stat);
 if(stat == MS::kSuccess)
 MGlobal::displayInfo( "created duplicate" );
 else
 continue;

 stat = parentNode.addChild(duplicate);

 MFnTransform dupTrans(duplicate);
 MFnTransform origTrans(dagPathList);
 dupTrans.set(origTrans.transformationMatrix());
 if(stat == MS::kSuccess)
 MGlobal::displayInfo( "added child" );
 else
 continue;

 stat = parentNode.removeChild(instNode.object());

 if(stat == MS::kSuccess)
 MGlobal::displayInfo( "removed original" );
 else
 continue;

 
 MDGModifier Modifier;
 stat = Modifier.deleteNode(instNode.object());
 if(stat == MS::kSuccess)
 MGlobal::displayInfo( "deleted original" );
 else
 continue;

I create a duplicate, add it to the instanced object's parent, set the transform, remove the old object and delete it. All looks good in the scene. I click on an instance shape and see many things light up in the view. However when I save the file out it is still the same whopping size! Is there something else I need to do?
Any help or advice much appreciated.