In fashion terms, Petite refers to height, not weight.
In French, petite means small, so people are sometimes confused by petite sizing. Many people think Petite sizing means skinny girls, and while slender women can be petite, so can plus size women. Petite refers to women under 5’4″, (162.56 cm) and doesn’t have anything to do with weight. Petite clothing can come in sizes from 00P to 14P, 16P, or even 18P.
There is also Petite Plus size clothing, or “women’s petite”. Most petite plus clothing comes in sizes from 16WP or 18WP to 24WP or 26WP.
Just like Tall sizes have longer leg lengths, arms and torsos, petite sizes are scaled down to be better proportioned on a shorter frame. Generally the shoulders are narrower, the arms and legs are shorter, and many designers also make the rise in pants shorter, so that pants are high waisted when they’re meant to be hip huggers. The area between the shoulder and bust is shorter, so that the dart go towards the nipple, not the navel.
Some ladies are true proportional petites, and some women may have longer legs and a short torso, or the other way around, and may be better off trying on both sizes, and wear regular sizes as tops, and petite sizes in bottoms, or vice versa. Sometimes even tall women can wear a petite size, depending on their proportions.
Some labels have patternmakers make petite sizes from scratch, and some places just “pink it and shrink it” which isn’t always a great fit. Petites aren’t just shrunken versions of regular sizes. They may still be muscly, or have large breasts, or look like regular height women in photos, so shrinking a pattern overall usually means arms that are too tight, pants that don’t fit over your bottom, or with buttons pulling at the bust.
A good label will also make clothing that actually flatters a petite frame. Just because you find a petite size of a dress you like, doesn’t mean it will be flattering. If all they’ve done is shrink a maxi dress with horizontal stripes, it might “fit” but it might not be flattering. That’s why you won’t find a petite version of everything in a store. Good labels will stick to making petite versions of things that will look good, like small patterns, vertical stripes, solid colors, etc.
Always make sure you look at the size charts when buying online, and if you can try things on in store to get a feel of a labels sizes, then you will know what works best for you later if you want to shop online.
So next time you’re out shopping, look for the petite section. Try on the same item in both sizes, and see which fits better. Don’t feel bad if you go up a size in the petites. They’re typically a half to one inch narrower in the waist, so you might be a 4R but 6P. Nobody else can see the label, so get what looks good in the mirror, not what size you think you should be.