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
Calvin Klein
Calvin Klein Women's Solid Strapless One Piece Swimsuit - Black - XL
Calvin Klein
Calvin Klein Women's Icon Shine One-Piece Swimsuit - Black - L
Calvin Klein
Calvin Klein Women's Icon Shine One-Piece Swimsuit - Neutral - M
Calvin Klein
Calvin Klein Women's Solid Strapless One Piece Swimsuit - Red - S
Calvin Klein
Calvin Klein Women's CK Icon One Piece Swimsuit - Black - M
Gap
Kids Cross-Back One-Piece Swimsuit
Gap
Baby & Toddler One-Piece Swimsuit
Gap
Convertible Bandeau Swimsuit
Gap
Scoop Neck Swimsuit
ASOS
ASOS DESIGN Charlene textured swimsuit with scoop neck and cut out in mint
ASOS
ASOS DESIGN asymmetric swimsuit in micro waffle knit texture with corsage detail in cream
ASOS
ASOS DESIGN contrast binding rib knit scoop swimsuit in navy
ASOS
ASOS DESIGN Kim ruched swimsuit in slate gray
ASOS
ASOS DESIGN seersucker scoop swimsuit with giant bow back detail in blue gingham
ASOS
ASOS DESIGN Tara super plunge swimsuit with tie detail in black
ASOS
ASOS DESIGN Hera asymmetric swimsuit in floral
ASOS
ASOS DESIGN Holly bandeau swimsuit with bead detail in seafoam green
ASOS
ASOS DESIGN Petite seersucker scoop swimsuit with giant bow back detail in blue gingham
ASOS
ASOS DESIGN Eloise textured cut out swimsuit with raffia trim in beige
ASOS
ASOS DESIGN Harriet adjustable straps swimsuit with hammered silver tone trim in blue