One aspect to consider is how much your breasts project forward from your body. Smaller breasts tend to project less and be wider on the torso. Venusian*Glow has a great post about what bras are good for shallow breasts with a broad base. More pendulous breasts tend to project quite a bit, especially when supported.
Imagine a rectangle that is 8" wide and 2" deep (for an area of 16" squared). This would related to a shallow breast shape, assuming you are looking down and your own breasts. :) A rectangle of 4" wide and 4" deep is more even, but still has an area of 16" squared. A rectangle of 2" wide and 8" deep STILL has an area of 16" squared, but this breast shape would have a lot more projection. So while the bra size may be the same (say, 30FF or 36H), different bras will have different shapes to accommodate different kinds of breasts.
But how do you find out what shapes different bras have? Bratabase has a TON of measurements for bras, but at this point, it is mostly just raw data. I decided to put that data to use, determining which bras are more shallow and which have more projection.
As you can see, Victoria's Secret bras are VERY shallow. So if you find that your breasts are getting smushed too close to your chest, you might look for something further down the diagram.
I will hopefully be adding more styles as more bras get measured in Bratabase. Add your bras today to help out!