That line by Duchesse de Polignac about how "the servants take over the house" seems pretty prophetic when you realise the context.
Fridge Logic: With all of the cake that the court, especially Marie Antoinette, eats - wouldn't you expect them to be fatter?
Some ladies were actually pretty plump.
And we only have paintings to go on- a an artist on commission is hardly going to make his employer look frumpy.
In truth, Marie-Antoinette was rather a light eater, and before her first child's birth, she was a very active dancer, spending most of her nights at balls, which may have helped with losing some weight.