I think the majority of clothes QVC sell are made against a size 12 model and then just sized up pro rata. Most plus sized clothes, the base sizes would be based on a much larger size or maybe two if they’re going upwards of a size 26. This allows for bigger back and bust, arm width and especially the rise in trousers.
I am a short, larger lady dont have an enormous bust but do have enormous hips and thighs. For some reason it’s deemed that if you’re fat, you’re 6ft tall with arms that come down to your knees! Fortunately I make a lot of my clothes and now how to adjust patterns accordingly. When I do buy clothes, I know which manufacturers actually design for plus sizes and therefore give a better fit. The only QVC presenter who knows about sizing is Julia Robert’s who is au fait with dressmaking and knitting.