Or look at her gut for a good few seconds with a disgusted look, then look up and stare at her in the eyes and say '"I am quite sure that I only need half of a sandwich." You might want to add "Half for now and half for later"  Saying something like that is nothing she can go to HR about, and if she does you could just play stupid and act all concerned about her #SelfEsteem and #WellBeing because you don't want her to feel #Bullied