Your bra cups should be large enough to prevent breast tissue from bulging or spilling out over the neckline and/or armhole edges. You'll know if your bra cups are too large if they have wrinkling, or are clearly larger than both of your breasts.
Here's how to test yourself:
Raise-Your-Arms Test
After fastening yourself into your bra, raise your arms. Does any breast tissue fall out from under the bottom of your cup? Does your bra slide up your body? If so, your cup may be too small and/or your bra band may be too big. You need to tweak your bra sizing because you are still not in the right bra size. Your correct size bra should stay put and have no movement when you are lifting your arms.
The Stoop, Swoop & Scoop Test
To know if you have the right size bra, you need to be sure you are “in” your bra. This is especially important for D cup sizes and larger
Here’s what you need to do:
1.) Stoop or lean forward from the waist and let your breasts drop into the cups
2.) Swoop – with the opposite hand, you gently bring your breast tissue forward from under your arms to make sure all your breast tissue is in your bra’s cups.
3.) Scoop your breast tissue up and into the underwire from under your bra band
Now, stand up straight. You might see some “fluffing” or a little breast tissue overflow on top of the cups and between your breasts. With each of your pointing fingers, start in the center and run them along the top edges of your cups. This usually gets rid of this bit of overflow. If it doesn't and you still have breast spillage, you need to go up one or maybe even 2 more cup sizes. If your bra style is a balconette or demi style, this might not be the right style if you don't like your look.
For a proper fitting, I suggest going to Victoria's Secret or a lingerie store and getting professionally fitted. That is the only way to be 100% sure of your true size.
If you press on the underwire at the side of your breast, you should always feel your ribs, not your breast. If you feel your breast, the garment is too small.
If you have any other questions, please feel free to contact me.