Thomas R Jefferys wrote:

Whether he _is_ a black piglet or not is a matter of interpretation, but I
do think that Takahashi-sensei did mean a piglet and not a small pig if she
chose "tun" rather than "xiaozhu," considering that she can construct
rather clever puns in Chinese (re: Shampoo = "Mountain Chest" :)~ and
Mousse = "Eye Obstruct" 8) ), it's obvious to me that she knows _exactly_
what she's doing. QED.

Personally I am inclined to think that the piglet does age, but only
in proportion to Ryoga's human aging process, and thus very slowly
for a pig.  In five years he might be quite a substantial little
porker.  But that certainly looks like an immature (rather than midget)
pig to me.  This is, of course, yet another clue for Akane to miss,
that P-Chan should have reached young adulthood by the end of the
series and and still just a piglet.