It's never really stated how old she becomes, at least not that I'm aware of, so she could still be in her teen years, hence explaining her actions. When she says 'I think I've grown a little', it could just be a little joke. It's quite obvious she's grown, a lot, but she's at that age where a little time results in a lot of growth. Cecil and the others are fully grown, so whatever aging goes on there wouldnt be as obvious. It's never stated how long Cecil's entire adventure takes... it could have been a few months between the time they lost Rydia and the time they met her in the underground at least. So, if Cecil and co. only spent at most a day or two in the Land of Summons, the changes wouldnt be obvious, most especially because they're all fully grown. They dont suffer a lot of dramatic change at that age.

I agree with Takara, I believe in full mental and physical growth, but there is no way to find a definitive answer for it.