Choosing not to wear a bra is a conscious decision and while it may be uncommon amongst full bust women it's still a choice that many make for aesthetic, comfort or health reasons. Choosing to wear the wrong bra is a less conscious decision. It's less of a choice and more of a last resort born out of a western norm of wearing bras and a lack of choice born out of most women only having access to 32+ A-D bras. Besides it being uncommon for busty women to go braless there are also some physical and health concerns about being busty and braless.
Lots of people think that women who don't wear a bra are doing themselves damage but there is actually very little data on whether low impact daily activities actually cause damage to your breasts and skin. Many women simply don't see going braless as an option because it's too painful to be unsupported (the larger your bust, the greater the gravitational pull on the ligaments in your breasts and this can be very painful). There is more information about the impact of not wearing a bra when working out. I wouldn't recommend that anyone works out without a bra because the impact on the ligaments and milk ducts inside your breasts can cause strain that is painful.
There is a potential rash/irritation hazard to your skin when not wearing a bra because moisture can get trapped between your breasts and your torso and prevent air circulating, especially if you're sat for long periods. Some women finding nipple chafing too painful to go without bras and don't like adhesive covers. On the whole though there is very little evidence that going braless is bad for your health (breast sag is primarily dictated by your genes, diet and skin care - for example, over exposure to the sun can cause your skin to become thinner and sag). On the flip side, there is a wealth of evidence about the damage a badly fitting bra can do to your health.
The most common mistake with bra fit is wearing a bra that is too big for you in the band and too small in the cup. A band that's too big for you can cause shoulder and neck pain, back ache, headaches, shoulder dents, chafing, skin irritations and bruising. Wearing a cup that's too small means that your breast tissue is often squished against other skin tissue which traps moisture (a perfect breeding ground for bacteria rashes) and prevents air circulating around your skin - all skin prefers to breathe.
When I see women wearing the wrong bra it breaks my heart because I know that it's not a choice, it's a last resort that's not empowering or comfortable. When I see a woman choosing not to wear a bra I see a conscious decision that is far better for your health than subjecting your breasts to the problems caused by the wrong cup, band and style. This is why the conversation about bra fit outside of the A-D paradigm is so important because millions of women still don't know they don't have to resign themselves to painful fit and they don't have to abandon bras, they can choose to be comfortable and happy in their bras with the right size and fit. xx