According to Items and Sizes Small and Medium creatures seem to be able to use Small and Medium weapons interchangeably without any penalty, and they can use Large weapons but they gain the Clumsy 1 condition.
The Bulk rules in this chapter are for Small and Medium creatures, as the items are made for creatures of those sizes.
In most cases, Small or Medium creatures can wield a Large weapon, though it’s unwieldy, giving them the clumsy 1 condition, and the larger size is canceled by the difficulty of swinging the weapon, so it grants no special benefit.
Items of Different Sizes also specifies:
Creatures of sizes other than Small or Medium need items appropriate to their size.
I’ve got several questions related to inappropriately sized weapons:
- What happens if a Large creature tries to use a Medium weapon?
- What happens if a Small/Medium creature tries to use a Tiny weapon?
- What happens if a Small/Medium creature tries to use a Huge or bigger weapon?
And all other possible permutations of these questions.
Then, there is no mention about damage at all, is it the same for all sizes? It seems absurd that a Tiny hammer does the same damage as a Gargantuan hammer.