Remember that in JA's time, the word "fat" was not as weighted with negative connotations as it is today. I seem to recall a reference to a "fat, healthy child" somewhere. Indeed, as noted by other posters in this thread, a rounded figure (i.e. no skinny arms, no collarbones showing) was considered a sign of health (and presumable fertility). In an age without antibiotics, if a very thin person got sick, she might not withstand the weight loss of an extended illness with high fever.