The author, a cringeworthy old matron from Paris' rareified 16th arrondissement, maintained that French women remain svelte by eating small portions and taking the stairs instead of the lift.
In my book* (*denotes gratuitous, self-serving plug), I argue that the reason French women are generally so slim is because of the nervous energy they expend worrying who their husband is sleeping with** (**denotes gross generalisation about an entire nation's apparent lack of marital fidelity).

But the truth it seems, is altogether more pedestrian.
While wandering down my rue this morning, I noticed this poster in the window of my local pharmacie. It's an ad for a pair of tights that in my home country might euphemistically be referred to as "control tights". In the interests of calling a spade a spade, these tights are essentially a modern day girdle - an item of apparel whose sole purpose is to suck in a lady's tummy.
"Two sizes less, WITHOUT DIET (and) WITHOUT SPORT" declares the advertisement.
Non-French women of the world take note. You can stop beating yourself up over why you appear to packing a few more pounds than your average French counterpart. The fact is, you're not. You're just not cunning (or weight-obsessed) enough to hide them behind a pair of super-tight stockings.