There are tools, like screw-drivers, that make better small throwers than most of the ones on the market. That's because the tools are heavier! Good small throwers are certainly possible to make, and sometimes you even find them. For example I have this integral boot knife that is only 6" long but it is a great thrower. That's because, as an integral, its reletively heavy.
Its pretty easy to make a small thrower that is heavy enough from any small piece of steel. Another option I've used is to take a commercial thrower - like the smallest Hibbens, and solder a small, flattened out, piece of lead (like a fishing sinker) to each side of the knife right at the center of gravity. That gives them a bit more mass. Another one I know lots of people like is just a simple piece of rebar cut to what ever length you like with a point filed at one (shuriken) or both (bo-shuriken) ends.
LOL)