When you divide something up and single out one portion of it for consideration, that portion is called a "share".
When we compare shares we talk about their sizes. "Small" and "tiny" are sizes. "Few" is not a size, but a quantity. It tells how many, not how big, so it isn't appropriate for a description of a share. "Low" tells either height, or (in this case probably) number. Again how many. It isn't a size, so it won't idiomatically describe a share.
For size adjectives, you have a wide range to choose from, all the way from "enormous" to "microscopic", including "small", "tiny", and all the suggestions already given by others.
"Little" can also be a size, so it might be OK to use in a description of a share, but it sounds a little odd here for 2 reasons:
1) It is a little bit informal for this context where we are discussing global GDP outputs, and
2) "Little" can also be an adjective for a small quantity of an uncountable noun (E.g. "The desert region has very little water."; "Tom has very little money.")
In this context "little" might, at first hearing, sound incorrect. So I think native English users would probably avoid it in this context, and pick out some other adjective of size.