iPhone charger plug gets hot

I have noticed that the plug which goes into my charging socket on my iPhone gets hot when charging. This question seems to get asked frequently on this and other forums, but the answer is always seems to be about the charger itself. Which is not what is being asked - what I am talking about is the plug at the other end of the cable which goes in to phone itself. Is this getting hot something to be concerned about?

I can fully understand why the charge itself should heat up, but not why the plug should. Is this normal?