The code generated for this line,

is expanded to this for instance for jsonFormat21:

    def write(p: T) = {
      val fields = new collection.mutable.ListBuffer[(String, JsValue)]
      fields.sizeHint(20 * 21)
      fields ++= productElement2Field[P1](fieldName1, p, 0)
      ...

I don't think ListBuffer actually uses the size hint so it's not that bad but if you ever switched to a different kind of collection that did (and actually side note: why not use Map.newBuilder instead?) it would be quite problematic.