It's not an error, there is just no general rule.
Sorry, but that's not possible, at least for well-written rules. If someone asks, "What determines the damage a projectile weapon deals, the projectile launcher, or the ammunition?" The reply should not be, "It depends."
That is the question here. Either the size of the bow is what determines the amount of damage or it isn't. It's a clear cut example of the rules contradicting themselves. Reduce Person tells us exactly what determines the damage for a projectile weapon. And then Enlarge Person /Righteous Might goes completely against it.
Again, all I want is an answer to a simple question. What determines the base damage of a projectile weapon? This should not be a difficult question to answer, and it certainly wasn't in 3E.
The 3.5 Reduce Person and Enlarge Person spell descriptions.
The fact that every single weapons table I can find shows damage values on the bow/crossbow/sling line and not on the ammunition line.
The table entries are the general rule, for lack of text in the description of projectile weapons, and the totally uniform response by the rules confirms it to be so.
I guess because the different spells enlarge you with varying degrees of physical improvement. They still all at least improve your strength score. And all 3 all seem to manage to make a medium longsword deal 2d6 damage, even despite those minor differences, which is the more relevant aspect to look at anyway.
Comparing magical boons to my question is also apples to oranges. My question wasn't even to what magnitude do the spells alter the base weapon damage of a projectile weapon. Merely what component of a projectile weapon determines the damage.
ThThe fact that every single weapons table I can find shows damage values on the bow/crossbow/sling line and not on the ammunition line.