It’s not nonsense though, there’s a lot of women wearing the wrong bra size due to poor fitting and out dated measuring. You might be wearing a 34 DD but that doesn’t mean a smaller band and larger cups won’t fit better. If someone’s rib cage measure 30” with a tape measure pulled tight then an elasticated 30” band bra will fit providing the cup size is right. If the band fits but feels tight and leaves red marks it’s far more likely that the cups are too small and breast tissue is being pushed into the band stealing the space.
There’s basic checks to do to see if your bra fits properly. Put it on backwards without putting your arms in the straps making sure it sits right up where it should. Pull it out, if you can pull more than an inch or so your band is too big. The support comes from the band not the straps. If the band is right then you check the cups, swooping and scooping from as far back under your arm as you can. If all your breast tissue fits in the cup then great, if you have overspill the cup is too small. If the band feels too tight on the right way, the cups are too small. There are a few exceptions for example swimmers with broader backs, but for the majority of the population bra bands will be dress size plus no more than 20” which also assumes the bra you are trying is true to size.
If you have tried all that above and your bras fit properly the great, you are the exception to the “rule” most of us fit. If women are comfortable in their bra size whether it fits “properly” or not then that’s also fine but there are so many people who do not know that they are still being sold bras inches too big with cups too small.
We have been conditioned to think adding 4” to a modern bra band is right, it’s not. We are conditioned to think anything above a D cup is huge, its’s not. A D cup means your boobs stick out 4” above your rib cage measurement, that’s tiny and if people are saying they think that’s big then I would put money on them being able to get a better fitting bra and needing a bigger cup size.