Adding the solution in case someone needs this in the future:

 FlxTween.tween(paddle, { "scale.y": 2 }, 0.3, { onComplete: tweenComplete.bind(_, paddle) });

 private function tweenComplete(tween:FlxTween, paddle:Paddle):Void
 {
     paddle.updateHitbox();
 }

@dean said in updateHitbox not working after scaling with FlxTween:

{ onComplete: paddle.updateHitbox });

Thanks, unfortunately I get this error:

source/PowerUp.hx:54: characters 52-85 : Void -> Void should be Null<flixel.tweens.TweenCallback>
source/PowerUp.hx:54: characters 52-85 : Void -> Void should be flixel.tweens.TweenCallback
source/PowerUp.hx:54: characters 52-85 : Void -> Void should be flixel.tweens.FlxTween -> Void
source/PowerUp.hx:54: characters 52-85 : For optional function argument 'Options'

Still having trouble understanding "Void."

Hello, here is my function when a power up collides with my Pong paddle:

public function paddlePowerUp(powerUP:PowerUp, paddle:Paddle)
{
    kill();
    FlxTween.tween(paddle, {"scale.y": 2}, 0.3, {});
    //paddle.setGraphicSize(Std.int(paddle.width), Std.int(paddle.height) * 2);
    paddle.updateHitbox();
}

If I do setGraphic without the tween the hitbox will expand, but not with the FlxTween. Any thoughts?