Basically it means she's more aerodynamic. with large breasts she'd catch wind and drag by having a slender lithe body type the wind doesn't drag on her body, sort of like how swimmers shave so that their hair doesn't drag in the water during races.
It's not even the profile that is the issue. One of the most basic laws of motion is that for every action there is an equal and opposite reaction, and in atmosphere that opposite reaction is extreme friction that can even take it's toll on metal. Not to mention the fact that at those speeds she wouldn't be able to see or... you know... breath.
Breathing is overrated. Just watch zombies, they don't breath, vampires neither, the Forsaken at WoW...
But yeah, motion laws are bitches... i remember this guy that became cutted in half by a bridge column. a thick diamond shaped column, but he was kind of fast riding a Ducati (you know how fast should be going to be ripped apart like a butchered pig) and lost control on a boulevard. left side of the body fell to one street, right side to the another, and the balls are still sticked at the column, people says (no, i'm not going there looking for someone's balls).