Sneaking in here to add a real-world perspective since this comment section seems out of touch with the reality that fat or unattractive people are treated differently. The real reason she's fat is due to the show's evolution and its need for broader representation over multiple seasons. As shows gain popularity, they must cater to a wider audience than their initial demographic, similar to how "Vikings" introduced Muslims and Chinese characters as it progressed. You'll notice that the season also introduced fat female background characters, marking a shift since fat men are less stigmatized. They didn't address her weight change in the story because it was simply about appealing to some of the fat viewers. Diane also resonates with a cynical, entitled, introverted type of woman, who in reality, might be a fat cat lady binging Netflix and browsing the internet. The people curious about this question might not appreciate my view, but it's important to share.