I think a 12 has become a large since clothes sizes became bigger, so that a 12 is no longer really a 12 (iyswim).
For example, at my 18th Birthday, I was 5'4", weighed abut 7st and wore a size 12 skirt from D Perkins(which I still own!). I am now in my thirties, weigh more than 2 stone heavier and I'm still a size 12.
In fact, I used to own a similar style skirt, which I also bought from D Perkins in the late 90s, which had a bigger waistline, even though it was only a size 10.
I think its called vanity sizing, and happenned a lot in the 90s. Certainly, I remember in the later 90s having some older Next clothes which had different european clothes sizes to the newer ones, even though they were the same british sizes.
I got this from wikipedia (which probably explains it better than me!)
Vanity sizing
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Vanity sizing, also known as size inflation is used to refer to the phenomenon of ready-to-wear clothing of the same nominal size becoming larger over time. Vanity sizing tends to occur in places where clothing sizes are not standardized, such as the United States,[1]
"It is generally acknowledged that clothing of the same nominal size has become larger over the years.[2][3][4][5] In 2003, a study that measured over 1,011 pairs of women's pants found that more expensive brands tended to be larger than cheaper ones of the same nominal size.[6]
The increasing dimensions of garments of the same nominal size has caused some designers to introduce size 0, 00, or subzero sizes."