Are you kidding me? Look at the size of her head in relation to her body. For that matter, look at her FEET! They used Kairi's KH1 model on her for CoM, and the KH2 model in KH2.
Still don't believe me? Watch the CoM scene on Destiny Islands where she turns around and becomes Kairi in Sora's eyes. Then look at her standing hand-in-hand with Kairi in TWTNW.
They are the SAME HEIGHT. Namine matches Kairi EXACTLY at any point in her existence.
I do believe you, I've seen the FMV, and I've noticed the difference.
But she's not a natural nobody, thus it could be the reason for her aging.
If all nobodies aged, wouldn't you have seen Roxas in a different light?
He'd have the same character model as KH1 sora but he didn't now did he?