Munchies:
Gotta remember people come in various shapes and sizes. Even at 400 lbs, an arm can look drastically different. If you aren't sure what body shape you want, go look at Google images.
Remember there are non-feedist people out there who are 400 lbs. In fact, there are weight loss websites out there with a database if pictures of people. You can pop in a height and weight, and it will spit out the closest match.
As for me, personally, I never include specific measurements unless it's important to the story. It's a real pet peeve of mine when people tell and don't show. Also ... so many male authors will say things like "She was so fat. She was 5'6", 130 lbs, and had huge C cups," and it's just an average-size woman no one in their right mind would even call chubby. Or particularly busty.
ValtheFA:
Oh, for sure, I try my best to keep that in mind, it's not all-encompassing, it's why I said a [general idea from measuring, I'm sorry if I gave that impression.
Sometimes I find that you can paint the picture, sometimes to help with framing the gradual weight gain over a long period of time, it's good to define the shapes at times so all of the characters feel like they are different sizes despite those similar weight thresholds. I searched a lot for the base sizes of my characters for fanfiction, not of my own design, and tried to thoughtfully think how the weight would affect them based on their body shape and how the adipose would regulate into varying sizes.
I will say in my own settings, I try to be less analytical and cold with my descriptions, but I can only describe someone's size so often when you hit the 25k mark with the same story, even with a very large cast. It gets even more difficult when doing an anime setting and the characters at base size are already at extreme proportions, that adding to them can feel a bit strange, to say the least.
I haven't thought of looking at weight loss databases to check though, that might be helpful in gaining more perspective on what I'm looking for. The bbw.wiki is really hit-and-miss for those details. Again, I appreciate your help and insight, too good to keep a frame of mind for that, and getting those reminders when looking back at my own work to do so is greatly appreciated.
To be clear, I am not against including measurements, but only when it makes sense to do so. For example, if a feeder is measuring someone, for example. But I like to be more creative about it. For example, someone breaks a chair with a 400 lbs weight limit, or their hips brush against a door that's 60 inches across.
But the thing you gotta remember is that how you imagine something will not be how the reader will imagine it. So you can't get too bogged down with details.
Personally, I like to give a basic description of a character and focus on the
experience of being fat. Things like discovering new rolls, relearning your center of gravity, or outgrowing old clothes. That way you don't have to worry about being too precise, but the reader can better immerse themselves in your world.