Tesla has 25% margins because they are in a different segment. If Tesla sold a large number of very cheap cars they would not be able to command 25% margins. If you lop off the top portion of VW's sales and look at their margins they will be much higher than the margins on the smaller stuff. But in absolute numbers those figures certainly help and they also help to protect VWs income stream during slow years.

Their premium brands tend to be hit very hard during those years (Porsche, for instance).