It has been proven without a doubt that a larger 19mm carb on a polini setup can easily pull a polini puch to over 1mph per cc. reliably and seize free for thousands of miles.
however this chart from dellorto would rate a 21 carb too large for a polini, given a polini produces 6hp. And a 19 right towards the upper edge.
I have seen the 21 work on some bikes but I think the 19 is a bit better, more velocity = more torque.
Then again I think some of our polinis are pulling over 6hp ;)