It's hard to measure this with just bust size alone. We need to talk about total body type.
Pinkie Pie would be at the top, thick thighs and a large rack from all those sweets.
Applejack would be next, but with a larger frame in general. More Muscular and larger built than the others.
The next three get mixed depending on how you mix them.
Rarity could either be 3rd or 5th. With a full figured body type she would be 3rd with an hourglass figure. As a size zero model she would be 5th. Either way, the ration of boobs to body would be seemingly bigger than the next step up.
Fluttershy and Twilight compete for the upper between Rarity. Fluttershy as a nurturing type vs Fluttershy as a hippie chick to see if she could be out the chocolate and energy drink physique of an academic.
Last is Rainbow as the toned body type of an athlete. Nothing wrong with that though, because that slim muscular body has a lot of appeal.