I was pretty unimpressed with this plotline for precisely the reasons you point out. We spend a quarter of the pilot with Kate, and literally the only thing we learn about her that isn’t related to her weight is that she has a twin brother. She seems to have no interests, no friends, no job, no life outside of being miserable about her weight.
By the way, having recently seen Chris Sullivan in Stranger Things and The Knick, I’m pretty sure he’s wearing a fat suit in this episode, or perhaps being shot and lit in such a way as to make him look a lot heavier than he actually is. Which may suggest that the show didn’t think the actor was big enough to play the character they wanted, or that part of his story will involve weight loss down to the actor’s actual size.