Patterns generally say to fit bust 34 or 36 etc however the woman with a 36″ bust could have a very different shape to the woman standing beside her with the same measurement. Look at bra sizes, a woman who is measured with a full bust of 36″ could be a 36A/14A or a 32D/10D or even a 28FF/6FF (yes that is a real size). The frame size isn’t taken into account if you just use the bust measurement.
If however you measure your back and front separately the woman in the 36A may find her back is 17.5″ and her front is 18.5″, the woman in the 28FF would be more likely to have a back measurement of 14″ and a front measurement 22″. Should they be wearing the same size? No. The petite girl needs to make a much smaller size and adjust the front to fit. If she has a flat tummy she would need a full bust adjustment, if she also has a tummy she may have a size difference between front and back at her waist and hip too which would mean choosing a different size front to the size she chooses for the back. The trick is then to use the vertical measurements from only one size so the pieces match and ensure you reduce enough for the shoulders to match.
This photo shows what happens if you choose a size that you fit into without taking the differences into account. The side seams pull forward pulling the whole thing out of shape so that there is enough room all round.
Remember we are all different but patterns are made to fit an average. If you want something that fits you well, you need to adjust the pattern to your own measurements.