"Judge me from a size, do you?"
I pledged to only judge by the math, never by the cover!
Magic is separate, and relates to the power of the caster rather than the size of the caster. It makes sense that way to me. I wouldn't want the small races to be utterly screwed on damage all the time for everything, nor would i want giants and dragons to do more magic damage from size when they already do more physical damage.
An RPG isn't about making sense, it's about following rules. How could we possibly teleport otherwise? Can you imagine the anatomic's horror of being a centaur? Have you consider that a centaur's brain, being the same as a regular human while having more body parts to manage, will have less room for potential, thus making invalid the "fact" that centaurs are wise?
Siege weapons, however, are physical and mundane weapons. Their damage obviously scales with size. Think of them as a creature with a natural weapon. The size of the object is the size of the creature, the damage is based on the creature having that natural weapon at that size.
The damages of a weapon does scale with size. I never disagree this point. Well, that's not entirely true: the creature (the sword that is) will do the damage of a creature of the upper size: a M sword is actually an S item,
just as written here.
My point was that siege weapon are actually miswritten and that
the damages doesn't scale like they should do, by rules that is. If you disagree on that point and want to scale, you would have to scale special ammunitions too, and alchemist's bomb while we are at it.
Any houserule or errata would
obviously correct that. And any normally functioning GM would put this errata right away. But this thread's name is "Interesting stuff in Pathfinder", and I think that pointing out that, still, by rules,
siege weapons don't scale with size is an interesting thing. That also means that your now Very Small Balista, Light will still require the same amount of people to reload and aim than when it was a L item, because the "Load" requirement is written and doesn't depend of the size of the siege weapon, only of what is written in "Table: Ranged Siege Engines".
And if you are stil not convinced, just look at the table of siege weapon and the one for equipment: the siege weapons have only one "Dmg" column, and that is because those weapons aren't designed to be made in other size. You can now look absolutely everywhere in any Paizo stuff. When damages scales there will always be two "Dmg (X)" columns, while non-scalling will only have one. Same for the feats giving natural weapons.
If you want a G Ballista you will create a Gate Breaker, not a Ballista, Light. Whereas your crossbow is a crossbow, whatever its size. A player cannot, by any means, create a Ballista, Light that isn't a L item, otherwise he will design a completely new item, a new entry that is. The entry will have new values, scaled by the chart you linked and houserule for "Crew - Aim - Load" since there's no rule for that.
-- Edit:
Honestly, I kind of like the idea of scaling spells up or down based on size. It's always bugged me that an enormous giant or a tiny pixie produces spells with the same area and damage as a regular Joe--especially when the reach and damage of their melee weapons is scaled to their size.
Granted, that would make pixie casters somewhat less dangerous and things like dragons or ogre magi a hell of a lot more frightening, but honestly I think that's more of a feature than a bug--especially when it helps preserve the suspension of disbelief. I mean, what giant would bother with "area" spells that don't even affect the entire space of a creature their size?
Whoops, didn't saw you. Well, Dragons bother to cast AMF while the description stipulates that some parts of their body will be left out! That would also means that D or lesser creature are totally useless. That would also means that any alchemist can come to a pond, put the regular amount of reagents, create a ([lbs_pound]/[lbs_potion]) * [actual_bomb_dices] damage and then throw a monster in it, destroying its very core. I honestly thing that, at this point, even a god would be crushed by receiving an ocean of bomb in the face. Well, as a DM I would have a minute of silence for the poor boy. Can you imagine the weight of the Earth's ocean? Because I can't!
Ok, I did the math: it looks like there is 1.1*e(12) liter of water on Earth. A vial of potion (filled) is 1 lbs, while the same vial is considered 0 when empty. One liter of water is 1 kg = 2.2 lbs. So there is 1.1*e(12)*2.2 lbs of water. let's say the weight of the reagent is acceptably null.
With 1L of water, you can do 2.2 vials. So the total number of vials doable on Earth is: 2.2 * 1.1 * e(12) = 2.42 * e(12).
A lvl 20 alchemist does 1d6 + 10d6 by vials. If the damage scale you now have nd6 (I'm sorry, I have no data for a weapon of a size of 1.100 km long) + (2.42 * e(13))d6 damage!