Weapons and Armor are exceptions. This is true for mundane and magic weapons. Weapon size determines things like damage, and there's a bunch of rules for it:
Weapon Size: Every weapon has a size category. This designation
indicates the size of the creature for which the weapon was designed.
A weapon's size category isn't the same as its size as an object.
Instead, a weapon's size category is keyed to the size of the intended
wielder. In general, a light weapon is an object two size categories
smaller than the wielder, a one-handed weapon is an object one size
category smaller than the wielder, and a two-handed weapon is an
object of the same size category as the wielder.
Inappropriately Sized Weapons: A creature can't make optimum use of a
weapon that isn't properly sized for it. A cumulative –2 penalty
applies on attack rolls for each size category of difference between
the size of its intended wielder and the size of its actual wielder.
If the creature isn't proficient with the weapon, a –4 nonproficiency
penalty also applies.
The measure of how much effort it takes to use a weapon (whether the
weapon is designated as a light, one-handed, or two-handed weapon for
a particular wielder) is altered by one step for each size category of
difference between the wielder's size and the size of the creature for
which the weapon was designed. For example, a Small creature would
wield a Medium one-handed weapon as a two-handed weapon. If a weapon's
designation would be changed to something other than light,
one-handed, or two-handed by this alteration, the creature can't wield
the weapon at all.
Armor doesn't work differently based on size, but its costs and weights change. See the table "Armor for Unusual Creatures" on the equipment page.
That these two have their own rules for sizing and are not mentioned in the rule about magic items resizing makes it clear that they don't automatically resize. In addition, the rules for creating magic weapons say this:
Weapons for Unusually Sized Creatures: The cost of weapons for
creatures who are neither Small nor Medium varies (see Equipment). The
cost of the masterwork quality and any magical enhancement remains the
same.
When you're making magic weapons or armor, you have to pay the extra cost to make the base item for large (or larger) creatures. When making something like a ring or cloak, you don't have to worry about that because they resize.
(I don't know if Pathfinder has it, but 3.5 had a "sizing" magic weapon property that would allow a weapon to resize to its wielder.)
