Since arrows for a small bow weigh less than those for a large bow, does that mean you need to specifically find (or buy) "small" arrows?
If so, would one assume that magic arrows found on (& used by) small monsters would be sized (only) for small bows?
We've always assumed arrows were one-size-fits-all, but then the difference in weight doesn't make sense.
Well, you could always pick up one of the small bows when you pick up the small arrows...
I'd probably allow you to use the small arrows but they would do small arrow damage even when fired from a full-sized bow, even a str-adjusted bow since you could't pull the bow all the way back.
Or heck, I'd probably just ignore the difference in size as too much accounting work...
But by RAW, arrows for different sized bows are NOT interchangable?
I guess by RAW bows come in small, medium, etc... but I don't see any RAW about arrows. Logic would suggest that small bows shoot small arrows, but I don't think ammunition falls into size categories, only weapons.
So by RAW, they probably ARE interchangeable.
A "small" sized arrow would generally just be a shorter shaft. The other bits are all the same. If you're worried about magic items or looted ammunition just make sure someone in the party has craft: bow and it could easily be ruled that you can freely take the heads and fletching off of one arrow and put them on another.
Also, I've always liked the idea that magic items resize, lending wieght to the power of them while also making accounting easier.