On Tue, 13 May 1997, T. H. Tiger wrote:

I think one example you have to take into consideration is P-chan
young pigs grow at a phenomenal rate and if he aged in his cursed 
form he should be at least four or five times his actual size. 
even if you argue that he is only aging at a human rate he should have
doubled in size by the end of the manga.

Whether that would have any bearing on Ranma is up for debate however.

  The correct answer is.. Takahashi was blatently inconsistent about this
entire idea.  Use whatever works, because there are theories on both sides
of the issue.  Like exactly how much water it takes for tranformations to
take place.. use whatever you want, there is no *right* or *wrong* amount.

<For the hell of it>
  My take on P-Chan:  P-Chan *has* his natural size.  He just happens to
be of a species of a minature pig.  Therefore he grows no bigger.
*Animal* forms do age and gain size, the perfect example being Pantsu
Tarou, who was dipped in the spring of drowned bull/crane/eel when he was
an infant by Happosai.