Two people buy the same swimsuit in the same colour, one size apart. One of them wears it for three summers and recommends it to everyone she swims with. The other spends the afternoon pulling a strap back onto her shoulder and quietly decides the brand is not for her. Nothing was wrong with the fabric, and nothing was wrong with either body. What went wrong happened at a drawing table months before either suit was cut. The word for it is grading, and it is the least discussed step in how clothing is made.
What grading actually is
A garment begins as a pattern cut in one base size, usually a small or medium. That pattern is fitted, adjusted and signed off, and it becomes the reference for the whole style. Grading is the set of rules that turns it into every other size in the run: each piece expanded or reduced by a defined amount at defined points, written into a grade rule table the factory follows. Done well, it is a form of design in its own right. Done carelessly, it is arithmetic applied to a shape that does not obey arithmetic.
Bodies do not scale in one direction
The easiest way to build a size run is to assume a larger body is a smaller body multiplied. Add a couple of centimetres to the bust, the waist and the hip, keep everything else the same, and move on. It is fast, it is cheap, and it is the reason so much swimwear fits like a compromise. Real bodies change unevenly as measurements increase. Torso length grows, but not at the rate circumference does. Shoulder width and back width grow more slowly than the bust. The distance from shoulder to underbust, which decides whether a one-piece sits or strains, follows its own curve entirely. A suit that ignores those relationships fits the size it was designed on and drifts further from correct with every step away from it.
What lazy grading feels like on the body
- 01A one-piece that pulls down at the shoulders or between the legs in the larger sizes, because the body of the suit got wider but never longer.
- 02Straps that slip off in the smaller sizes, because strap placement stayed where it sat on the base pattern while the back narrowed.
- 03A back that gapes or cuts, since back width was graded at the same rate as the bust rather than its own.
- 04A leg line that reads high and sporty at one size and unexpectedly revealing at the next.
- 05Bust shaping that drifts outward as sizes increase, so the seams that should give lift land beside the bust rather than under it.
- 06Lining and elastic cut to a single length across several sizes, overriding whatever the shell was supposed to do.
A size run is not one pattern photocopied larger. It is the same garment redrawn, again and again, for a body that changes shape as it changes size.
Why swim punishes it more than anything else
A dress can absorb a grading error. There is ease built into it, fabric hanging away from the body, a belt or a drape to take up the slack. A swimsuit has none of that. It is held on by tension, cut smaller than the body it covers and relying on the fabric being under a precise amount of load. Slightly too much and it digs, marks and rides up as you move. Slightly too little and it sags and goes translucent where it loses its stretch. Compression sharpens this further, because holding only works when the stretch at each point is what the designer intended. Then it gets wet, gains weight, and pulls down on every one of those decisions.
What fitting the whole run changes
The alternative is slower and more expensive, which is why it is rarer. Instead of fitting one sample figure and grading outwards on paper, a brand fits real bodies at several points across the run: at the smallest size, at the largest, and at the sizes between where proportions shift most. Each fitting is a chance to correct the grade rather than confirm it. Strap position moves. Torso length is added where it is actually needed. The leg line is redrawn so the same cut reads the same way at every size instead of creeping higher as the pattern grows. None of that is visible in a product photograph, which is precisely why it is the first thing cut when a collection has to arrive quickly.
How RIVA handles it
RIVA grades every shape across the full XS to XL run on real figures rather than a single sample size, so the lift and coverage hold at the ends of the run and not only in the middle. Each shape is tested wet, stretched and dived in before it earns a place; a fit that only works dry and standing still is not a fit. The sculpting Italian jersey the collection is built from is double-lined, and that lining is part of the fit rather than an afterthought laid over it. Every product page carries a fit note worth reading before you choose between two sizes, and the compression means a new suit should feel snug, then settle within a wear or two. If the size is wrong, exchanges are free within thirty days on unworn pieces.