I wear anywhere from size 2 to size 8 depending on the brand and half the time even within the brand I have to go between sizes. This makes shopping for clothes a pain because I hate having to try things on in the fitting room before I buy them. I've noticed men's clothes uses a consistent sizing system for pants that doesn't vary from brand to brand because it's based on actual measurements of the waist and inseam. How come women's clothes can't use a logical system like this? It would make so much more sense than the crappy system we use now.
Another reason, besides the ones previously listed, is that women's clothing tends to be much tigher and form-fitting than guy's clothes, but women's bodies have more complicated and varied shapes, and lots more areas to measure and to fit right.
Guys buy pants, and it's all "waist and inseam", but girls not only also have an extra measurement (hips), but she also wants a certain fit in her ass, and may have to try several brands or lines to find ones with a fit she likes. Same with shirts; you have boobs that require more work to fit, especially with tighter clothes or dresses. For a guy, most stuff is one size measurement (M or XL or whatever), and even for a dress shirt, it is neck size and chest size, but no need to worry about detains beyond those.
Men's clothes, for started, aren't always that accurately sized; there can be varience from store to store just like women's clothes.
That being said - I think one of the main problems with sizing is that different clothes are made to fit in different ways - and people don't always picture themselves wearing something the way it's designed to fit. And there's a lot of variance in shape for women. For example - a lot of girls can wear a smaller size on their top than their bottom - which can then make buying say a play-suit or something like that difficult.
And some places don't have regiment measurements for things. Like a lot of places will have a pretty similar idea of what waist size, makes a dress size - but they don't account for arms or something like that - so to find a top that fits well - they'd have to buy sizes up to fit on their arms, even though it fits the body perfectly - or something like that.
I am 120 and 5'3 and also wear any where from a size 2 to a size 8 :)
women's clothes are more complicated though, we have curves. clothes have to fit our waist to hip ratio, ass, and thighs. women's body shapes are more complicated than men's. I think that is part of it.