[ There are technically two surprises here, though naturally one overshadows the other. He does look surprised when Xion says she's only a year old, but that's part of what he has to reconcile (or more accurately, brush away) as a part of the strangeness of this place. She's not human, and she's also extremely young, certainly looking impossibly older. But with the comparison of being a doll, he supposes that a doll is shaped to have a certain appearance. Age could factor into that.

Yet even with that surprise that certainly seems impossible to him, it's not what he focuses on, naturally. He addresses the smaller, more personable surprise. ]

Then happy birthday, Xion. I'm sorry that your celebration was— [ He trails off, seeming to think of a polite way to put it ] Perhaps not what one would hope for a birthday party.