I think of women as fat at about 230 pounds. Less than that can be curvy, attractive, "thicc", whatever, but 230 is more than just chubby. BBW.
At 300 pounds your pants may come from ordinary retailers but you're wearing them tight. "Plus size" sometimes doesn't cut it. You wear out chairs you sit in a lot if they aren't particularly sturdy. You haven't nearly topped out the underwear that ordinary retailers sell. You make hardly any adjustments in life, and there are lots of people your size everywhere, but almost nobody considers you normal size.
At 400, you require a high capacity scale. You pants come from specialty retailers. You have to be careful about sitting in some chairs, and you may have broken a few. Your underwear is still garden variety, but sexy underwear comes from specialty retailers. SSBBW. Lots of people this size on the streets, but people think you are uncommonly large.
At 500 chairs that aren't particularly sturdy will complain even when you're careful. You've certainly broken a few. You don't fit in some. Jeans are hard to find but they're still available to you. Your stretch pants come from just a few retailers. Your sexy underwear comes from specialty retailers, but granny panties are still available everywhere. We don't see a lot of people like you, but there are a lot of you who just don't get out much.
At 600 you have to be careful about what chairs you'll sit in, and you'll wear out just about any chair eventually. You don't wear jeans. You're wearing the largest sizes from even the specialty retailers. You are truly uncommon, but there is still a market for your clothes. USSBBW.
The heaviest woman I see regularly weighs almost 700. She's still able to buy underwear and dresses, but she's twenty inches bigger around than the size charts, so these things don't fit the way the manufacturer intends. Shirts are still no problem, but her favorite "shirts" are actually short dresses that she pulls down over her hips and belly and fit like a long shirt. She wears them with stretch pants that she has made for her, because there are no leggings in her size. She can still get "sweat pants" though she's twenty inches bigger than the size charts for them. She sits on two chairs and walks through doorways sideways. We almost never sees other people her size. We know how much she weighs only because of freight scales, as even her doctor can't weigh her.
The heaviest woman I've ever seen is about 800 pounds. She, too, gets into "sweat pants" and off-the-shelf underwear, but they fit funny and require constant effort to pull up over her behind. It looks like she's going to burst out of them, all the way down to her ankles, but they do make clothes this big. It takes a small team of people, a motorized scooter, and a van to get her out of the house. I've seen her sit on single chairs and damage them, and I've seen her just about sink a two-person pool float. A rare delight to see her.