I remember reading somewhere that mature youkai (as in, really long-lived ones) are able to control their physical appearance to an extent. It's probable Tewi can make herself appear more womanly (in the sense of what we think about when the average adult body is concerned), but prefers her short stature as her default human form. Easier to pull pranks and escape the chasing prankees after all.
Of course, I'm not complaining about a busty Tewi. I'll gladly take either/or.
