In Jane Austen's Pride and Prejudice, why is Elizabeth pleased to find that Miss de Bourgh is "thin and small"?

By Chapter 28 of Jane Austen's Pride and Prejudice when Elizabeth sees Miss Anne de Bourgh for the first time, Elizabeth has already formed a very negative opinion of Darcy. She has already decided he is an excessively prideful man who mistreats everyone he feels is beneath him. Her opinion of Darcy has especially been formed by Mr. Wickham's account of Darcy, from whom she also learns that it is expected of Darcy to...

By Chapter 28 of Jane Austen's Pride and Prejudice when Elizabeth sees Miss Anne de Bourgh for the first time, Elizabeth has already formed a very negative opinion of Darcy. She has already decided he is an excessively prideful man who mistreats everyone he feels is beneath him. Her opinion of Darcy has especially been formed by Mr. Wickham's account of Darcy, from whom she also learns that it is expected of Darcy to marry Miss Anne de Bourgh.

In Chapter 16, we learn that, according to Wickham, Darcy disregarded his late father's will and refused to give Wickham the living at Pemberley promised by the late Mr. Darcy. Wickham's argument is that Darcy refused out of a general, intense dislike of Wickham fueled by jealousy; Darcy was jealous of his own father's devoted fondness for Wickham. When Elizabeth hears this, she draws the conclusion that Darcy is capable of "descending to such malicious revenge, such injustice, such inhumanity" (Ch. 16). On the same evening that Wickham informs Elizabeth of Darcy's alleged mistreatment of Wickham, Wickham also informs Elizabeth that Lady Catherine de Bourgh is the sister of Darcy's late mother and that Darcy is expected to marry Miss Anne de Bourgh to unite the Pemberley estate with the Rosings estate.

Hence, when Elizabeth sees Miss de Bourgh for the first time while visiting Charlotte at Hunsford, the living at Rosings Park, Elizabeth is very pleased to see that Miss de Bourgh is "thin and small." Elizabeth thinks to herself that Miss de Bourgh "looks sickly and cross," and because Elizabeth thinks Miss de Bourgh looks like such an unpleasant person, she feels that Miss de Bourgh will make a very suitable wife for Darcy--the type of wife she feels Darcy deserves.