My first guess is that they measure a lab and a pointer, and scale everything from there. Maybe a Chi and a Cocker for some companies that cater to small dogs. Or they don't bother at all.
The 35-45lb dogs also suffer from sizing. Bigger than a Cocker, smaller than a Lab. BC are probably the most common dog in the range, and to make matters better, they're all over the place in body type. Toys are very common (x-small-small), retrievers are very common (large) so medium covers anything bigger than a Cocker and smaller than a Lab.
If dogs range from ~5 to 200 lbs, they need to be covered by more than five sizes (xs, s, m,l, xl). Sled dog harnesses are normally custom fit, I really wish some other jackets, backpacks and life jackets could be custom ordered.