Just to remind people - a size 12 has padding.
I know this, because I am a size 12, and I have a layer of padding all over me. Kate clearly does not have any padding in that photo of her modelling at uni.
I am 1.77 metres tall to her 1.75, so even though I am tall, I still have a layer of insulation at a size 12. If you put me in that sheer dress, I would look very different! 
She has always been slender, and if she has lost some weight since uni (understandable), it's nowhere near the vast amounts people try to make out it is.
I think Kate has a remarkable figure - tall, slender, narrow waist, catwalk-worthy legs, straight shoulders. So when I say it's not often I see anyone with her figure - it's for all of these reasons.
Not just because she's a size 6 - which is why many people on here are trying to claim they're just like her.
As she's arguably smaller than a size 6 for a start - and it's really not just her dress size - I do think people are flattering themselves.