This print is pretty self-explanitory. Regency Gowns simply don't flatter every figure. Compare Gillray's take on the new fashions of the late 1790s with Boilly's, where a dedicated follower of fashion (a merveilleuse) is mistaken for a prostitute because of the general skimpiness of her attire.
In the 1790s, fashion took a complete 360 from the poufs and panniers associated with Marie Antoinette. The French Republic wished to complete change French culture, and what better medium than fashion? Since the National Assembly wanted to build a republic the likes of which had only been found in ancient Rome, tailors and dress-makers took their cues from ancient Roman statues, which meant high waists, long, trailing drapery, and simpler, curlier hair-styles.
Since the French set the fashion, England followed, even though for most of the period when empire-waist dresses were popular, England and France were engaged in a series of long and vicious wars against each other.
Fashion, apparently, is the one import that continues despite naval blockades and Russian winters.