So, in this post I am going to go more into depth about how you can tell you are wearing the wrong bra size.
The band:
The band is the part of the bra that is supposed to give the most support to your breasts The band, NOT the straps. If you're wearing the correct band size, you won't have to over tighten the straps. A correct fitting band will be parallel to your waist/the ground. The band shuld be sitting just below your shoulder blades. The band should be going straight across, there should not be an 'arch'.
Here are some pics
Also, the band should be worn on the loosest setting not the tightest. You wear it on the loosest, so that when it starts to stretch out, you can gradually move it tighter. The only exception to this is if you're pregnant, then you wear it on the tightest, and loosen it as you progress in your pregnancy (because your rib cage expands). If you start out wearing your band on the tightest setting, what are you going to do when it stretches out?
The straps
As I said above the straps are not doing most of the work of the bra.
The straps should not be so tight that they're leaving indents in your shoulders, but they also shouldn't be so loose that they're falling off your shoulders.
The bigger your breasts are, the wider you want your straps to be. I don't have big breasts, and most of my bra straps are 6/8 inch wide. I have very few bras where the straps are 1/2 inch wide.
The gore
The gore is the piece in the middle that connects the 2 cups. If you are wearing a regular bra that fits correctly, the gore should be touching your skin, from top to bottom. The exceptions to this are: if you're wearing a sports bra, a bralette, or wireless bra. If either the bottom or top of the gore isn't touching your skin that means there's a fit issue with the cups, and possibly the band.
The upper left and middle pic illustrate what I mean
in the upper left pic the top of the gore isn't touching; the top middle pic the bottom of the gore isn't touching
The cups
In a correct fitting bra, there shouldn't be any boob bulging out of the bra, from the top, or the sides. When you put a bra on you should make sure all of your boob is in the bra (I'll get into this later).
If your bra look like this
it. does. not. fit.
both of the arrows are pointing to breast tissue that should be in the bra.
This is the same person in a better fitting bra.
The left pic shows the breast tissue bubbling over her cup.
If your bra looks like this, it's too small.
Side boob- you shouldn't have this if you're wearing the correct bra size/shape
the girl with the purple bra has wire that is sitting on breast tissue.
This bra is obviously too small in the cups
So the proper way to put a bra on:
put the bra on with the cups in front; you can be standing or leaning, but f you're new to this leaning is best, so your breast tissue is basically directed into the cup. You hook the bra closed from the back. Thi may be hard to do if you have 'coordination' issues, or if you're a bigger size and there's 4 or more columns of hooks. Then pull your straps up; Here you can
'swoop and scoop' to really make sure your breast tissue is in the cups. Then you adjust the straps.
Swooping and scooping is important to ensuring the cus fit correctly. You can put a bra on and not swoop and scoop, and think the bra fits, but once you swoop and scoop, you'll see that it doesn't (too small); likewise, you can have a bra that you think is too big (because there's space in the cups), but once you swoop and scoop the cups either fir, or are too small.
Here is a blog post about swooping and scooping with pictures
Here are 2 pics from the post
Most people will probably think this bra fits, but this is before she swooped and scooped, making sure ALL of her breast tissue is in the bra
This is after she swooped and scooped. You can obviously tell it is was too small.
And yes, this works for itty bittys as well
To swoop and scoop:
(your bra has to be on)
Pull any 'fat' you have on your sides (from your underarm forward); This may be breast tissue that has been displaced by wearing inccorect bras for years
Lift your boobs up, and make sure the wire is sitting in the spot where your boob attaches to your chest.
If you do this enough, you *may notice a change in you breast shape, because the tissue is going back to wear it belongs. This is called tissue migration. Some women in the bra fitting community don't believe this is a real thing, but if you've experienced it, you will know it does.