Please help me out and check my work; I'm terrible at math. So if I'm a Large minotaur archer with 18 Strength, a (large) Lifting Belt, and a (large) composite longbow, how many arrows can I carry before taking penalties?
L items, such as arrows, are treated as negligible. However, I'm probably using Large arrows. A single Large arrow is 1/10 Bulk, or L. So 10 Large Arrows are 1 Bulk, which is treated as L for my minotaur.
So...
05 base
04 Strength
01 lifting belt
==
10
x2 Large
==
20 Bulk Limit
-4 Large composite longbow
-1 Large lifting belt*
==
15 Bulk remaining
/10
==
150 Large arrows (+9 more for 159 total, since we ignore 9 L)
So, as a GM I'd say the arrows for your Large Longbow do indeed need to be Large.
10 Arrows have a Bulk of L for a Medium creature, so per Table 6-20 on page 270 of Player core, 10 Large arrows become 1 Bulk, and they cost 2 sp.
Per "Bulk Conversions for Different Sizes" on the same page, "A Large creature treats 10 items of 1 Bulk as 1 Bulk"
So for 1 bulk, you can carry 100 large arrows, just as a medium creature can carry 100 medium arrows for 1 bulk.
As for the belt, as I answered in your other thread there is no clear ruling on this, but I personally would say that yes, it grows up to 1 bulk. But again, Large creatures treat items of 1 Bulk as being L, so the impact on your bulk limit is the same.
I hope this helps.