Many years ago, the rich dined on horrendous fatty protein-rich diets, and spurned vegetables and most fruit. Hence they were fat. The poor couldn't afford to eat enough to keep body and soul together, so they were thin.
Now, the poor have to fill up with cheap carbs, e.g. pasta and spuds, so they get fat. The rich can afford healthy gourmet diets so while not thin exactly, they don't have the obesity levels of the poor.
I imagine they were more or less the same size during the War.