i dont think so......my friends nephew used to do baby modeling when he was younger and sometimes they would put him in girl clothes (not for pictures...lol) to make sure of the fit for that specific size.
i have found that weight doesnt seem to matter in clothing sizes. i actually find it pointless when the weight suggestion is listed on clothing tags. you dont see that on adult clothing...lol!
your twins wearing 2 different sizes is just how they are proportioned. i bet if you had boy/boy or girl/ girl twins, they would probably wear different sizes too.
A structured jacket is one that is fitted and shaped to the body through outer details such as shape defining darts and curving seams. A structured jacket holds its shape and is of a stiffer fabrication. Much of what makes a jacket structured is on the inside where you can't see it, between the fashion fabric and the lining. It will be interfaced with canvas, non woven materials, tapes and paddings while the outer seams will be pressed crisp and flat with no top stitching or edge-stitching. A structured jacket will look the same on you as it does on the hanger, nipping in at the waist and it will curve where you curve. A structured jacket is for dressy business, day and semi formal wear. You will always looked dressed up in a structured jacket. Most business suits are structured.
And unstructured jacket has few or no darts, the seams are not contoured to the body and it has little inner elements to hold the shape. An unstructured jacket will be soft, floppy and boxy looking while on the hanger and it will give the wearer a boxy shape. They are less rigidly finished and pressed and there will be some top and edge stitching on them. Unstructured jackets are for casual day wear or for a sportier and less buttoned up look. These jackets always give the wearer a more casual look. If an unstructured jacket is too large and shapeless it can also look sloppy. Sweater jackets, safari jackets and cotton jackets are unstructured jackets. Sorry no pics!