When women switches a badly fitting bra to one in her correct bra size, a very common reaction they get is along the lies of "wow, you look slimmer" or "have you lost weight?" or "your waist looks more defined". A correctly-fitted bra can change the entire silhouette, taking it from shapeless to defined.
This is how that works:
In a badly-fitting bra the band is too loose and not supportive, making the breasts flop over and below the band. I tried to show it in the little schematic drawing below. The difference here is just the fit, when I say a "supportive bra" or band I don't mean a special high-tech bra or a sports bra, just a regular bra but in the correct size. So the drawings could represent the same bra model, once in the wrong size and once in the right size.
When the badly-supported breasts flop over like the drawing on the right, they optically merge with the belly and create a general impression of a heavy midsection. (You might also notice that with a loose band the point of contact between breast and ribcage below is big, and this often leads to chafing. And to clothes not fitting right.) But when the breasts are lifted with a snug supportive band, they reveal what is often the slimmest part of the torso -- the ribcage just below the breasts.
This is not just true for bigger-busted ladies. I am small-busted but quite short-waisted (here is how to calculate that), and my waist isn't very defined. This means that if my breasts weren't lifted, I wouldn't have much waist at all (see my metamorphosis photos).
Breasts that sit higher also draw the eye upwards and make the whole figure look perkier and more youthful. Sag gives a tired look to the figure, and also drags down the fabric of clothes, creating unflattering lines.
Also, a well-fitting bra smooths out the figure by dealing with armpit and back rolls. Yes, I said back rolls. Here is an interesting post about how a tighter band can minimise back fat (see how the looser band of the white bra digs into the back much more than that of the snugger black bra). Sounds counter-intuitive but actually isn't.
There is another reason why well-fitting bras improve the figure:
A typical woman's reaction to an unsupportive bra is to tighten the straps, hoping to lift the cups that way. The straps only pull the back of the band even higher, and the front droops even more. A see-saw effect if you will. The straps start either chafing or slipping off (usually both) and often the woman ends up hunching her shoulders to avoid that.
In a well-fitted bra the snug band carries most of the weight of the breasts and stays in place, so the straps stop being a bother. There is no more tension on the shoulders. And women unconsciously stand straighter when they feel they have a bra that lifts and supports.
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