Weather balloons are tiny, yes, but they aren't tracked by radar anymore. Generally, it's a set of meteorological instruments and a GPS to track the location. It's a bit easier to get that to work, as you no longer need to make sure your radar works, but rather, just that the GPS is working before you let go. That said, payload is substantially smaller than these things (<1 kg for current tech).
I assume this would be slightly more clear as "aren't tracked by radar anymore [by their controllers]" -- who does what tracking and how, of course, wildly varies. (Not trying to be overly pedantic, just saying out loud my internal thought reply.)
