What's the difference in weight between a Tiny, Diminutive, or Fine creature's item and a Medium creature's item of the same variety? For example, I know that when you have a Human sized backpack it weighs 2 lbs, but when you have a Small sized backpack it weighs .5 lbs (1/4 the weight). So would you follow this progression down the line? tiny backpack would be 1/16th the weight, or (1/8 lbs) 2 ounces, and a diminutive backpack would be 1/64 the weight, or (1/32 lbs) or .5 ounces? Some diminutive creatures have strength scores of 4-6 and every ounce counts (especially since coin weights don't scale with creature size :-P ). I mean an unladen swallow is at least diminutive if not fine and a coconut is nearly a pound, so...
Volume is cubic, so as we size up or down, we alter volume in factors or multiple of 8; since we assume mass is proportional to volume, we adjust it likewise.
So a diminutive version of something should be 1/8 the weight of its tiny size, 1/64 its small size, and 1/512 of its medium size... meaning a diminutive backpack is basically weightless, yes.
So a small backpack *should* be 1/4 lb, not 1/2! Of course, we aren't surprised by the occasional rules inconsistency.
The funny thing is, smaller creatures can actually carry relatively more because the masses corresponding to their sizes decreases geometrically while their strength decreases linearly: if you take a heavily encumbered character and cast reduce person on them they can become less encumbered, even though their strength drops by two, because the stuff they're carrying becomes 1/8th as heavy!