Namine having the same bust size as Kairi makes sense; she is her nobody, so she has the same body as her.

Xion having a smaller bust is because she's memories of Kairi from KH1.

Aqua is likely the oldest, even before the timeskip after BBS. She's also a Keyblade Master, so of course she's the best.

Oh, and I'd be Larxene would be around Xion's size, maybe larger.

Who?