I know that a size 12 for example can be a different size and shape in different shops, but I think it gets even worse in that Next or M&S for example say that their size 12 waists are to fit a 32 inch was, but actually the waist in the clothes is more like 34 or 36 inches, so even more massive.
Interesting from Gablians link that waists have increased by much more than hips and busts since the 1950s and that clothes manufacturers are cutting accordingly. This explains why nothing fits my 37/29/39 figure. My body is stuck in the 1950s 
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In modern sizing, I have something like a size 12 top, size 10 waist and size 14 hips, and something like size 8 shoulders in M&S, whose dresses appear to be cut to fit massively over the shoulders 
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The reason why Zara trousers are very long apparently is that, while Spanish women are on average shorter than say northern Europeans, they like to wear very high heels with their trousers going down over them, hence the above average length.
But I do think trousers have got longer in general over the years, and that women have got noticeably taller. I'm 5'7'' and as a teenager/early 20s, rarely saw women taller than me, it was something that I really noticed.
And trousers we almost always too short, or just long enough to fit my 32 inch inside leg even in the long leg length size.
But now, 20 odd years later, I often see women quite a bit taller than me and even regular length trousers are often 32/33 inch long, with long being 34 or 36 and actually too long.
This thread may also explain why, on average, women spend longer than men clothes shopping. It's not that we purposely set out to do so, it just takes us longer to find things that fit 
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I can't imagine the average man going into a shop and thinking 'I wonder which version of inches they use in here'? Could I be a 32/34/36 inch waist? No, they just pick up the 34s and they will fit.