Into The Wild
Changing Size and Item Weight
There is no hard and fast rule for how weights change if a character changes size. The default that is stated in the equipment page is that item weights are given for Medium creatures (or in the case of saddles, a mount FOR a Medium creature, i.e. a Large creature). Items for Small creatures (or Medium sized mounts for Small creatures) weigh half and items for a Large creature (or Huge mounts for Large creatures) weigh double. There are some exceptions, such as containers that weigh 1/4th for Small (and no mention of what happens for large) and things like armor which has a fancy table.
Items in extra-dimensional space do NOT react to size changes of their owner (unless the owner is holding them and is inside the same extra-dimensional space!), but they will scale when retrieved. However, magic items (not weapons or armor) automatically change size to fit their wearer. Such items will RETAIN the natural size of their current wielder until someone else takes them. For example, a Small creature finds a Ring of Invisibility and it resizes to fit them. It STAYS small even if they take the ring off, until they give it to someone else. However, if they cast Reduce Person on themselves (to become Tiny), and they take the ring off and put it down, the ring will revert to SMALL (the natural size of the most recent wearer) since Reduce (and Enlarge) Person stop when items are removed. Also note: Items like Bags of Holding WILL change their weight based on size (this is primarily to not make Halflings unable to use a Bag of Holding type IV effectively)
The final catch: It seems silly to me that Magic weapons and armor do not resize to match their wielder/wearer, when mundane weapons/armor do. However, the rules are set up like this (I think technically any item like Bags of Holding or Aparatus of the Crab shouldn’t, but it is much easier to just say “not weapons or armor” since they are called out specifically). I may introduce a special Weapon/Armor upgrade, priced per a +1, that allows them to change size (this would allow characters who have magical means of changing size to properly utilize magic weapons at a cost).
A couple other random thoughts: Items like Trail Rations that are priced primarily by weight…. Since Small sized versions weigh half (or is it a quarter?) their price is likewise reduced (possibly rounded up to the nearest CP or SP for convenience).
NOTE: This is all subject to change, especially if you can provide counter arguments!