There are many reason that sizing is not the same through out the fashion industry. All clothing is made from patterns and there is not a uniform pattern that is used by all clothing manufacturers. Patterns have been developed by different manufactures through out the world for the various heights and shapes that women come in. In the past few years many clothing stores have been up sizing so what previously was their size 12 is now a size 8. They might get stock in that was using their old sizing measurements so that particular line will seem smaller than what they have in the rest of the store.
If the clothes are cut from really old sizing patterns from 30 years ago they will seem a great dealer smaller than usual. Anyone who buys vintage clothing knows that what was a size 6 in the 1950s is now considered a size 0 (or in some stores size 00) Known as vanity sizing it was a way to make women feel they stayed the same size as they aged and gained weight. In general North American women are much taller and much heavier than they were 30 years ago.
Another reason for a smaller size is a clothing manufacturer might try scrimping on the amount of fabric they use on a particular piece shaving a quarter of an inch here and there making it smaller than the size stated.
The clothes might be made in a country were the average size of their population is much smaller than the European or North American market or it might be made in a country where the women are in general more full figured and the cut is totally different. On the other hand a very cheap manufacter might make a sample size in only size 6 and if it is bought by a store size it up by adding half an inch to the measurements for every size it increases. What they end up getting are clothes in larger sizes that have gaping neck holes and huge sleeve openings. When clothes are fitted in larger sizes not everything expands, the neck on a size 4 dress and on a size 14 dress are not that much different, it is the chest, waist, hips and upper arms that have the largest change.
There are instances where the clothing is mistakenly labeled in size and it is not caught till after they are shipped to the stores and the only choice the store has is to send it straight to their clearance section.
Petite sections of the store are another area you can find great deals. A petite cut means that the distance from the waist to under arm is smaller than the regular cut. With some styles there is no defined area where the waist is, like in a flared coat. Lazy manufactures will just take the same size 10 regular flared coat and mark it as a size 12 petite, in many cases they will not even make it shorter.
As a savvy shopper you have to stop relying on the size label on a piece of clothing and make the mirror the final judge on whether you should be buying something. Not every woman fits the measurements of clothing perfectly. They might be slightly wider in the hip than a size 8 but going to a size 10 they think they don't fill out the bust. Not all clothes will fit perfectly off the rack and there are times (many times) that you will have to bring out the sewing machine to get a proper fit on a garment. We accept the fact that jeans are cut to different body shapes but can't accept that with everything else we buy is as well.
The reality of buying clothes is clothing size not only varies from store to store but brand to brand in the same store. The uniformity you would expect does not exist. Women do not want to buy clothes that are marked in sizes much larger than they are use to because it makes them feel as if they have gained weight. They also don't want to buy clothes that are marked in sizes much smaller than they are use to because they fear they will quickly out grow them.
So armed with the knowledge that sizes don't matter the next time you head off to a store with a wide assortment of clothes try to train your eye to tell if something is going to fit you. Start looking at the clothes that you would regularly expect to fit and guess before you try them on if they are going to be too snug, too loose or just right. When you feel comfortable about this start looking at clothes marked one or two sizes above and below you regular size. Don't forget to check the petite section as well and try on clothes a size or two above you regular size. A store will pretty quickly figure out that the size 14 dress is really a size 10 and that they will have to put it on sale to get rid of it.
Some of the best deals you will find will be in higher end stores that have clearance sections or clearance sales. One high end leather store had a $400. black leather swing coat on sale for $69 because the size label was XXL but any one could tell that it was no where near that and more likely a size L.
If you really have a problem dealing with buying clothes that are marked in larger sizes than you are use to or sizes considerably smaller than you are use to then a small pair of scissor might be a solution for you-so you can cut the size tags out of them when you bring them home.