There are a number of reasons why a bra won’t fit you and it is important to distinguish exactly what is that reason in order to address it on the next fitting attempt.
Doing a gross simplification, the two most important parts of a bra are the cups and the band (remember, “gross simplification”), and even though it might not be immediately obvious for many women when a band is too big, it is almost always easier to determine when a cup is “too big”.
Here’s my point, just spotting that your cup is “too big” is not enough to get a right fit on the next bra to try. “Too big” is not an appropriate term to describe how a bra fits.
These are some reasons how a cup can be “too big”:
If the cup is too wide for your breasts you will notice that the outer side of the underwire is reaching far into your back. Taking this step you risk now having some quad boob, if this is the case move to a different bra model all together.
If the cup is too tall, you will find that even though your breasts fill the bottom of the cups fine there will be empty cup fabric on top of your boobs.
First thing to try is to tighten the straps a bit, but remember not so much so they’re holding most of the weight of your breasts.
If you just go to a smaller cup, you might find that now the cup is too narrow for your breasts. It is always worth a shot but if it doesn’t work you will want to try a different cut of cup, a different style or to a brand that has shallower cups.
Too high on the sides
When this happens, it is best to just move to a different model, when a bra does this is usually related to how the model is cut, and if the cup fits but you’re getting poked under your armpit with the wire, going down a size will result in boob spillage.
These bras tend to work better with women with longer torsos or lower positioned breasts.
Too far between straps
Sometimes the straps might be positioned further apart than what your upper body actually is. The best sign is that the armhole of the bra is cutting into your arm. When this happens the cup might seem too big just because the arm pushes the bra forward and down to get some room. A smaller cup size might be too small or still have the same issues. If so just look for a different brand or different model.
If your cup is only too deep (after properly adjusting the straps), but the underwire encloses your breast nicely both on the center and on the side, then your best shot is to move to a model with shallower cups. Assuming the band fits right, if you just go down a cup size in the same model you will not risk either have the underwire sit on the side of your breasts or not having the center gore flat against you.
If the band is too big, probably just going down a band band size might be enough.
The exact opposite symptoms for a cup that is too small apply, as you can see in most cases when you are very close to the right fit, in almost all aspects but not one or another, then just moving cups might not cut it and just move on to a different model.
Remember that not all bras will work for all breasts, and your breast shape and breast style combination are as big of a factor as the bra’s label size to be a good fit for you.
PS: Thanks to Nesoa for the edits