This line from the letter made me feel sorry for Jane:

"I feel rather languid and solitary -- perhaps because I have a cold; but three years ago we were more animated with you and Harriot and Miss Sharpe."

I can totally empathize with feeling even lonelier in a large group than one might at home where there were actually fewer people. And summer colds are the worst, because the things that would make you feel better like a cozy blanket and hot tea are not that comforting when it's already hot outside. The second part of the line there, where Jane says it was more animated (and fun?) with Cassandra and two other friends there the last time makes it sound as though it's not the actual number of people gathered, but who they are.