John Singer Sargent worked with commissions. His reputation was based on his customers reviews so he had all the good reasons of this world to make the portraits of his customers look better than the customers actually looked like. There is no better way to make someone look better than s/he actually is, but achieve also the proper likeness, than giving to him or her a few more cms of height by drawing longer his or her legs.
It is the equivalent of a photoshop retouch in painting terms.
I bet that he made the waists of his female customers slimmer than they actually were and also their necks thinner and taller too. Not that much in order to look odd but enough to ensure that his clients would be satisfied.