With 25 years of experience in the garment industry, I can indeed confirm sizes are not created equally. Not only are they not equal from manufacturer to manufacturer - but even within a manufacturer's own labels the sizing can be and will be different.
Why?
A manufacturer fits to their "average" customer. So, everything starts out designed to a size medium (or a 6 or an 8) and everything is scaled up and down from there according to average measurement increments. A medium means nothing more than the middle of the size range offered by a given manufacturer for a particular line geared towards a particular customer.
One reason is demographics. For example, the higher end labels most definately have a scale that is geared small. Demographic research finds that the customer shopping at Banana and buying Ralph's Blue Label have more discresionary income, and those same demographics also say that those women are thinner. So, their medium (their average customer) is "smaller". Their size range is scaled accordingly.
Another reason is the target customer. Manufacturers fit to who they identify as their customer. If they are targeting a younger customer - that is who they are going to fit to. Gap, A&F, Hollister are good examples of someone targeting the youth....the teen/twentysomething customer. They haven't had babies (for the most part, lol) - they're not curvasious for the most part. These garments are not engineered for an older woman's body. Gap lost a lot of their older customers when they changed their fit...their response? They have a new line out - with different fittings - to reach the thirty and older crowd. Gap's Forthe & Towne size 6 is definately not equal to a Gap size 6. It's roomier and engineered to fit a body that has seen a little more gravity and life.
JJill is a favorite amongst older and non-Twiggy women because it is specifically designed for the 30something and up woman.
Lastly, sizing is evolutionary. It absolutely does change like people do. The average woman today in any market segment, is not the average woman of the 50's or 60's or 70's....now, (for the most part) we're taller, proportioned differently, and yes, weigh more. So, even though 10 years ago you were a size 8 (average), but are now 10 pounds heavier and still a size 8 - you're still average according to today's standards (as deemed by the specific manufacturer).
The denim dilemna....you could just be choosing the wrong fit or manufacturer. With jeans in particular - you need to kiss a lot of frogs before you find your prince. In addition to sizes getting mixed up in factories (on purpose and by accident, either because of sewing or shrinkage) denim designers have a specific niche when they build their product. You might find some jeans are are designed for women with thin thighs and long legs and some are specifically built for women with bigger hips and a rounder butt. This doesn't mean they are two entirely different sized women; they just have different builds. So, if a curvy woman happens to buy jeans engineered for a lean woman, then she's going to be forced to buy a bigger size and then deal with the gap in the waist. Conversely, if a thin woman buys curvy jeans, they won't fit in the right places, and will look awful....and, she'll complain about fit!
Manufacutuers rely heavily on feedback from customers....call their customer service dept or make a complaint at the store if you're adament about the sizes being messed up.
I know from experience, that with enough feedback, a manufacture will react. This happened with one of the lines we manufacture for Sears; when it was designed - they were aiming at one customer, but when (Sears) started getting all these calls about size issues - we and they realized they had totally missed the boat on who was buying the line! The size scale was changed accordingly!
. At least those sizes are standardized and they make wonderful presents. 
Now, I wish women's clothing would come standardized like men's.
Whoever said Dolly Parton should design clothes has the right idea!!!
Yeah, PS was pricey (at least for my budget), but at least I knew it would fit, so if I really needed something I could save up and get it there. Not that I usually liked the styles (a little old/business-y for me), but it was great for getting a few basics (like black dress pants).
