Because slimness is the desired look and the sort of woman who have obtained these positions in society tend to be quite driven and used to working hard to get what they want, the same drive and will power will be applied to body shape.
because if you are wealthy, you can give yourself a “treat” of a beauticians visit or another handbag, or a weekend in a posh spa, or a new dress - you don’t need your “treats” to be as cheap as a chocolate bar (or two).
because sample clothes only come in small sizes, if you want to wear the most recent (not yet in the shops) fashion, you need to be slim enough to get it in, and that’s a good incentive.
because if you are used to someone else cooking for you, it’s less of an event to eat out at a posh dinner, and the aim is usually to spend a couple of hours talking to the people around you, not eating.
because the quality and taste of the food is the primary thing, not the quantity.
because if you are surrounded by people who aren’t snacking or eating a lot, you tend to eat less.