Swimwear
Swimwear is cut with negative ease more aggressively than almost any other clothing category — the fabric's own stretch, not the cut, is what makes a suit fit snugly, which is why swimwear sizing frequently doesn't track a brand's regular clothing sizing even when the size numbers look the same. Reading the brand's own swim-specific size chart, rather than assuming your usual size, matters more here than almost anywhere else.
Construction details change support and coverage independently of size: a bonded or lined suit holds its shape and support more consistently than an unlined one, and underwire, shirring, or ruching each change how the identical size fits a given body. Two suits in the same size, same brand, can fit differently depending on which of these construction choices they use.
Fit also isn't static over time the way a cotton t-shirt's is: chlorine and salt water both degrade the elastane in swim fabric faster than normal wear does, so a suit that fit snugly in June can measurably loosen by the end of a season of regular pool or ocean use — not a defect, just a property of the fabric worth knowing before assuming a suit "stopped fitting."
Swimwear at retailers
COS
Contrast-Trim Swimsuit
COS
Asymmetric Double-Faced Swimsuit
COS
Twisted Textured Cutout Swimsuit
Gap
Cutout Wrap One-Piece Swimsuit
Gap
Scoop Neck Swimsuit
Gap
Scoop Neck Swimsuit
Gap
Cutout Wrap One-Piece Swimsuit
Gap
Cross-Back One-Piece Swimsuit
Gap
Kids Cross-Back One-Piece Swimsuit
Gap
Baby & Toddler One-Piece Swimsuit
COS
Bonded Swimsuit
Calvin Klein
Calvin Klein Women's CK Icon One Piece Swimsuit - Black - M
Gap
Kids Rash Guard One-Piece Swimsuit
COS
Bonded-Scuba V-Neck Swimsuit
COS
Color-Block Scuba Bandeau Swimsuit
COS
Asymmetric Double-Faced Swimsuit
COS
Twisted Textured Cutout Swimsuit
COS
Embellished Plunge-Neck Swimsuit
Old Navy
Ribbed V-Neck Halter One-Piece Swimsuit
Old Navy
Ribbed V-Neck Halter One-Piece Swimsuit