It is the most widely repeated piece of clothing advice there is, and for a jumper it is often correct. For a bra it is usually the opposite of correct, and for structured garments it introduces a new set of problems while solving the wrong one. The advice fails because comfort in clothing comes from where a garment sits, not from how much room is inside it.
Why a bigger bra is usually less comfortable
Support in a bra comes almost entirely from the band, which works by staying firm against the ribcage. A larger band is looser, so it slides, rides up at the back and transfers the load onto the straps. The result is shoulder pain and constant readjusting, which is precisely the discomfort the size increase was meant to fix. The correct move is often a smaller band with a larger cup, which keeps the volume and restores the anchor.
Band and cup are linked, not independent
Cup volume is relative to the band it sits on, so the same cup letter means different volumes on different bands. Sizing up on the band without adjusting the cup shrinks the effective volume, which is how people end up with a bra that is both loose and cutting in. Reading the full band and cup grid for the range concerned, of the kind published at https://just-my-size.com/, avoids that trap far more reliably than working from a remembered size.
Where sizing up genuinely helps
It works for outer layers that need to go over something else, for cotton items that will shrink in length, and for anything you will wear over a bulky base. It also helps where the fault is width rather than structure. Those are real cases, and they are narrower than the general advice suggests.
Where it makes things worse
On a top with a defined shoulder, a larger size drops the shoulder seam down the arm, which restricts movement rather than freeing it. On trousers, a larger waist with the same rise means the crotch hangs lower and the whole garment drags. On anything fitted, the extra fabric goes where it is not needed and the tight part stays tight.
- Bra too tight in the cup: keep the band, go up a cup.
- Bra riding up at the back: go down a band, up a cup.
- Top pulling at the armhole: change the style, not the size.
- Trousers tight at the hip but fine at the waist: check the rise before sizing up.
- Cotton tee that fits exactly: size up for shrinkage, that one is fair.
The related myth about wires
The same logic explains why people conclude that underwires do not suit them. A cup that is too small pushes the wire outwards onto soft tissue and it hurts within an hour. Going up a full band size makes it worse by loosening the anchor. The fix is a bigger cup, not a bigger everything.
How to test the advice on yourself
Order the size you calculate and the sister size, wear each for half an hour indoors and note where each one moves. The one that stays put is correct, even if it feels firmer at first. Firm and tight are not the same sensation, and it takes a day or two to tell them apart.
The rule worth keeping
Fix the specific problem rather than making the whole garment bigger. Almost every fit complaint is local, and a general size increase is a blunt response to a precise issue.
