bras CAN fit but don't have to cover all your breast to do so!
You can buy full cup, half cup, quarter cup and balcony style.
I like a half cup, balcony or plunge style with lots of graduated padding as I am a small 32C.
I measure something like 30-31 under my bust and used to buy 34s. I am now wearing 32 and have got used to this- at first it felt tight but not now.
All styles are different mind you- sometimes a 32 will not even fasten across the back so then I do have to go for a 34.
If the centr of the bra does notl lie flat on your ribc age, the cup is too small- this is because your breasts are pushing the bra away from your ribs.
Basically, the cups should be big enough to contain all your breasts, according ot the bra style, at the sides and the front- though if they are a half cup bra they won't. But even with half cup bras you should not have "muffin top breasts"! Your breasts should not literally spill over the cup edge, they should lie flat against it.
The way to find the right fit is to try them all on for size.
The support comes from the band- so if it is too loose you will not gt support. Hoiking up the shoulder straps will just give you sore and red shoulders.
If you measure under the breasts, that is your band size- forget the silly add 4 inches stuff.
Then go up a cup size for each inch across the breast- if it's the same- suchas 34 and 34, then you are an A cup or even AA, etc.
I measure 30-31 under and 34-35 over, so tend to wear a 32C or occasionally a 32 D in a small cupped bra.