It's about a lady in flesh.

Slim body is a modern standard of beauty originating from The West. Historical Russian belle is a sonsy woman having enough food to eat and thus originating from prosperous family and healthy to bear children. This is the historical logic explaining the lure of a thick body

Kustodiev reflected this buxom type of beauty on his famous paintings. The most popular is Merchant's Wife ("Купчиха за чаем").

While "кустодиевская красавица" is definitely positive you can also meet ironic "кустодиевская купчиха" referring to a sonsy woman in contemporary language.