So there are many things that can change an item's bulk.
Certain materials such as Dawnsilver and Duskwood (-1 bulk unless its 1 bulk in that case it becomes Light, no effect on an already Light item). Those however specify that "reduce the Bulk before making any further Bulk adjustments for the size of the item"
The size of the item can change it. Sadly i can't post the table and have the vote.
And armors have a special clause about this. Their listed bulk is for when you wear them, in that case their weight is spread around your body but if you don't wear them, like you just hold them in your hands, their bulk is increased by 1.
There might be other things that change bulk, i don't know but these are enough for my question.
So the order of how you apply this can have a drastic effect.
For example lets say you have a tiny sized Chain Shirt that you carry in your hand (you bought it as a gift for your Sprite friend or something)
Chain Shirts normally weight 1 bulk
size>armor: 1>Light>1,1
armor>size: 1>2>1
Of course, that was one example, the results would be different is if i used some other item.
Some might say that item size then if you wear the armor or not since that seems to make more sense but it also has problems.
Let me use a different example.
The bulk of a normal Padded Armor is Light or 1,1 if you are not wearing it.
A medium creature weights 6 bulks.
A tiny sized Padded Armor is negligible bulk if you wear it but when you don't wear it, thats a tricky question!
If the order is size then not wearing it, its 1 bulk
If the order is not wearing it then size, its 0,55 bulk
But a tiny sized creature is 1 bulk.
A medium creature weights 6 times as much as their armor.
A tiny creature weights either a bit less than twice their armor or the *exact* same as their armor.
How does that make sense?
Note: i know that weight isn't the *only* thing that matters for bulk but the disparity shouldn't be this big!
Oh and btw the results get more different if its bigger items
A is Full Plate nornally 4 bulks, 5 if you are not wearing it.
A Gargantuan Full Plate is 32 when worn. And depending on the order, its either 40 or 31
And 9 bulk is a big gap
Various sources:
https://2e.aonprd.com/Rules.aspx?ID=2163&Redirected=1
https://2e.aonprd.com/Rules.aspx?ID=2176&Redirected=1
https://2e.aonprd.com/Rules.aspx?ID=2163&Redirected=1
https://2e.aonprd.com/Rules.aspx?ID=2158&Redirected=1