Hi. I'm a fashion designer and one of my specialties is pattern cutting , fitting and grading to size. I understand how sizes work and who the size ranges are designed to fit. First of all the ranges of sizes for females Girls, Junior and Women's do not smoothly transition from one to another. So you don't go from a girls 14 or 16 into a junior 5 or women's 6. Also women's 6 and junior's 5 are cut to fit similar measurements but with different body proportions. Confusing? You bet! Let's break it all down in bite size chunks.
Girls sizes are cut to fit a body with little to no difference between chest waist and hip. They are cut to fit a pre-adolescent body of the typical girl. With no shaping between bust, waist and hip they will feel tight in the wrong places and loose in weird places if a developing teen or adult tries them on. And no, girls XL does NOT transition into a junior's XS. You would start with a Junior's size 7 or a M and work from there. Some find they go from girls XL to a junior 9 and this is 100% normal. Once a girl starts growing a bust line then she needs to move out of girls. This isn't a judgment call on her shape by the industry, this is to allow for the differences in the ages which girls reach puberty. In fact it's LESS of a judgment.
Junior sizes were originally called "young junior teen" Somewhere over the years the young and teen were dropped and only "junior" remained. These are cut to fit the adolescent developing body of the younger woman. The sizes are all marked in odd numbers - 3,5,7,9, and so on. If a garment has an odd number size it's a junior fit. The smaller size juniors overlap with the larger girl's sizes as I mentioned. Junior sizes are cut with an A or B cup bust, a five inch difference between waist and bust and a five to seven inch difference between waist and hip. These are cut to fit the less curvy body of a girl who is still growing and developing. These sizes fit a more-angular less-round body.
Women's sizes are cut to fit a fully developed, curved, well proportioned adult body. These clothes are cut with an B to C cup sized bust, ten or more inches difference between waist and bust, and approximately ten inches difference between waist and hip. These sizes are expressed as EVEN numbers -2,4,6,8 and so on. These fit the fully developed figure of the adult woman. even number sizes are less likely to gap in the back of the waist or pull across the thighs and seat because that extra ease is is built in.
Women's and juniors 100% overlap. Height is the commonality between sizes. They fit similar measurements in height but different proportions in girth. If you don't fit into junior sizes because the hips are small and the waist large it does NOT mean you need to lose weight. It means your body fits the proportions of Women's sizes better. It means you have a more shapely woman's body and not an angular teenager body. If a size 7 in juniors is too small in the hips and thighs and baggy in the waist it means you need even number sizes, not weight loss.
Don't shop by age. Everyone's body develops at a different rate and some teen never fit into junior sizes because they never have that angular phase -they go from girls sizes to even number women's sizes. And there's isn't a smooth flow between size groups. so yes, a junior 7 can be almost the sale as a girls 14 -but the girls size has no fitting allowance for developing breasts, a narrowing waist or developing hips. And there are many similarities between a a 10 and a 9, but one will fit better because your body proportions are more in line with the woman's fit rather than the junior fit. Buy according to fit and don't be upset if you are teen but and require a woman's fit in clothes.
Complicating matters even more is that there's no such thing as standardized sizes. A size 12 in one store will fit like a size 10 in a another while in a third store the size 9 might fit best. Do not focus on letters and numbers when buying clothes. The idea of something being "true to size" no longer exists. Always try things on before buying and buy according to fit and feel. If something fits then it doesn't matter if it's a S, XS, 5, 8 or any number. No one will even see the size number and they won't see if it's an even woman's number or odd junior number. All they will see is a garment that fits and flatters your body and the only thing you should care about is that the garment looks as fabulous as it feels -no matter what department you found it in.