Peri and Anne raise good points about the push towards tighter bands. My fitter explained this preference as follows:
Most women prefer to support and raise breast tissue so it aligns to the mid-point of the upper arm. This is the “traditional” bust position used by patternmakers and designers so tailored clothes tend to fit better when the bust is more or less in this vicinity. For many women, a loose band means the weight of the breast tissue positioned like this would be borne mainly by the shoulder straps. A tighter band relieves the pressure on the shoulders since the band can now function as a support for much of the weight. This alignment and weight redistribution, of course, is more noticeable on women with larger busts and more breast tissue.
The old way of measuring for band size—measure and add 4”—may have been a reasonable guide in the days when bras were made of heavy cotton and bands were much wider—often with 5 or 6 hooks—and extended to the upper rib cage. Cotton bras were notoriously prone to shrinking when washed and did not stretch when a person bent over. Having some ease allowed a woman to take a deep breath and move without discomfort. The typical bra of today uses much lighter, stretchy fabrics and narrow bands so the old 4” of ease would, nowadays, fit a woman in an extremely loose band.
For most women, using the exact measurement is a better starting point in figuring out band size since the built-in stretch of the fabric often provides sufficient ease for comfort. Different brands use different designs and fabrics so the stretch in one brand’s bra band can be quite different than the stretch in another brand’s band. That’s why “sister” sizes can be so helpful—if a band feels too constricting, going up a band size and decreasing a cup size may be a good compromise. Often, though, just like in any other garment, the underlying issue is the design, or brand, just doesn’t “get” that particular customer. In that case, restricting your search to just the one or two brands offered at a store doesn’t make any more sense in bras than it would in any other category of clothing.
To my mind, bra purchases are like buying shoes. A SA might measure my foot and tell me a shoe is my size, but only I can judge if I’m happy when that shoe is actually on my foot. Bras are the same. Our bodies are unique and only we, not some SA, can judge what feels comfortable, looks good, or fits our preferences.
