Monstrous Physique II allows you to take the form of Large Monstrous Humanoids, some of which have the Undersized Weapons property, which states: "the creature uses manufactured weapons as if it were one size category smaller than the creature's actual size."
However, the Polymorph rules state: "If your new form does not cause your equipment to meld into your form, the equipment resizes to match your new size."
Does this mean that if I were wielding a medium longsword and turned into a centaur, that the longsword would become sized for large creatures, and therefore I'd have to use it as a 2 handed weapon with -2 penalty for it being inappropriately sized? And that if I'd started with a greatsword, I couldn't use it at all?
Bump.
In this case, it's a small adjustment to say that the longsword or greatsword becomes the size appropriate to your new form, so it remains Medium.
I agree.
But this question is for a somewhat unpopular magus PC, so I'll probably keep as concretely to the RAW as I can; I just wanted to make certain I was reading things correctly.
That seems a rather poor rationale for the decision; the rules say it that equipment resizes to match your new form. The new form uses Medium weapons, therefore the gear resizes to be Medium (or more likely, remains the same)
Centaurs are size large. The rule says that equipment resizes to your new size.
I'd originally thought your equipment adjusted to your form as well, but my DM pointed out the wording and told me to post here. I guess the responses are on the side of RAI, and I'll let him know that, but without overwhelming response or rules evidence, I suspect he'll rule "against" me.
The effect of the ruling, is only that the magus need to spend a move action to drop and pick up the weapon. While that might mean that he looses the ability to full attack in one round, the drawback is minor. It is most likely not going to compensate for whatever makes the magus unpopular.
Weapon Cords?
Perhaps. I am not sure whether or a weapon hanging from a weapon cord counts as having left your possession.