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?
In KH2, we see him born as a Nobody in front of the mansion.
For about a minute.
What would be the point of going to all the trouble of making a model for a scene that short-lived? They've skimped before; the model of Namine that shines through Kairi at the end is the YOUNG version who's drawing at the beginning--even though we KNOW she should have changed because you can SEE it. (And you can tell that she doesn't match Kairi's build the way she did ingame; her eyes are bigger in her face, her chin is smaller, and her shoulders aren't as broad.)
There's also the fact that Roxas is Sora's exact height and build, despite the fact that Sora has aged a year since he ejected Roxas.
Agreed, that should probably change in 358/2 Days, but again I don't see them going to the trouble of showing the progression of time when, for one thing, we're only talking a year and they aren't going to change
that much.
But for the sake of the argument, let's say Nobodies
don't grow. Let's pretend Roxas got kicked out of Sora and just stayed the same, and Sora spent about...eh, ten years inside a plothole. Then they decide to fuse again.
Since Roxas hasn't aged, does that mean Sora's
body hasn't aged, despite the fact that it's been up and moving and
living (unlike Kairi's when her heart was first stolen)? How much sense does
that make?