Do you say this because the H!M perf? Because it's what I thought too
I think it's the perf view 16:9 itself lol Miki is already more tiny than Aya. And her hairdo... it's not helping but making a chuby face. I think it's all our eyes. She's hotto hotto. More Ayaya to hug 
It's also evident in the GAM PV, but not so much as compared to the H!M performance. I know in other countries (most especially Western ones), Aya would be the one considered skinny, but when she looks "big" next to Miki...now you kind of have to be worried for Miki.
And yes, the hairstyle isn't helping Aya either. She needs the long hair, kind of like Nacchi, so it doesn't make their face appear 'rounder'
I agree,she looks much whiter too,
Aya's never actually been "tan" though, she's had very fair skin ever since, unlike Miki.