This is why we have an Internet, by the way: to tell us things like this.
The reason the Barbie doll has to have such a small waist relative to the size of the breasts is that Barbie is designed to look good in doll clothes, and when you make doll clothes, you have to use normal fabrics, and you have to make seams and double the fabric over in a way that gets very bulky, especially around the waist. The doll’s unreal proportions become much more real if you put the clothes on.
Mind you, this answer is a lot less satisfying to a certain type of mind than the alternative answer Toy companies hate women and want them to be unhappy. In my (admittedly cynical and probably shallow) opinion, that attitude is mostly restricted to people in the fashion industry…