I've noticed that the driver does not mind if maxPoolSize gets set to a value lesser than minPoolSize:

static String uri = "mongodb://adminx:123@mubuntu:27017/test?authSource=admin&minPoolSize=1000&maxPoolSize=10";

I believe that the behaviour in such a configuration is undefined.

For comparison, the Java driver throws if the same connection string is used (which I believe is the correct behaviour):

java.lang.IllegalStateException: state should be: maxSize >= minSize