Sprite Kit - SKShapeNode产生了奇怪的大小

Question

我正在尝试为玩家精灵生成多个平台以便跳转。

这是我目前的代码：

var mainGroundAdded = false //starting ground for the player to run on

func addGround() {
    let groundSize = CGSizeMake(self.frame.width/2, 50)
    let ground = SKShapeNode(rectOfSize: groundSize)
    ground.fillColor = SKColor.brownColor()
    ground.name = "ground"

    ground.physicsBody = SKPhysicsBody(rectangleOfSize: groundSize)
    ground.physicsBody.affectedByGravity = false
    ground.physicsBody.dynamic = false
    ground.physicsBody.restitution = 0.0
    ground.physicsBody.categoryBitMask = groundCategory
    ground.physicsBody.collisionBitMask = rCategory

    if (mainGroundAdded == false) {
        ground.position = CGPointMake(0, 0)
        mainGroundAdded = true
    }
    else {
        ground.position = CGPointMake(self.frame.width+ground.frame.width, 0)
    }
    self.addChild(ground)

    let move: SKAction = SKAction.moveToX(-ground.frame.width, duration: 4)
    let remove = SKAction.removeFromParent()
    ground.runAction(SKAction.sequence([move, remove]))
}

这就是我不断产生地面的方式

override func update(currentTime: CFTimeInterval) {
    /* Called before each frame is rendered */
    var timeSinceLast: CFTimeInterval = currentTime - self.lastUpdateTimeInterval
    self.lastUpdateTimeInterval = currentTime
    if (timeSinceLast > 1) {
        timeSinceLast = 1.0 / 60.0
        self.lastUpdateTimeInterval = currentTime
    }

    self.updateWithTimeSinceLastUpdate(timeSinceLast)
}

func updateWithTimeSinceLastUpdate(timeSinceLast: CFTimeInterval) {
    self.lastSpawnTimeInterval = self.lastSpawnTimeInterval + timeSinceLast
    if (self.lastSpawnTimeInterval > 1.5) {
        self.lastSpawnTimeInterval = 0
        self.addGround()
    }
}

地面没有产生的其他地面那么大。我该如何解决这个问题？