SpriteKitでテクスチャー同士の間に隙間が空いてしまう。

お世話になります。

textureとして、不透明な四角いpng画像を使っています。
＜画像２＞のように、画像ファイルは色のついた部分だけなのですが、spriteにすると、
並んだ時に＜画像１＞のように隙間が空いてしまいます。
隙間を設定するプロパティなどあるのでしょうか？

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＜画像１＞

[GameScene.swift]

import SpriteKit
import CoreMotion

class GameScene: SKScene, SKPhysicsContactDelegate {

    let ballCount = 3
    let accelerationSpeed : CGFloat = 5
    let textureCount : UInt32 = 4

    var textures = [SKTexture]()

    let texture_left = SKTexture(imageNamed: "Frame_left")
    let texture_right = SKTexture(imageNamed: "Frame_right")

    var motionManager: CMMotionManager!

    var ballXposition : Double = 0
    var ballYposition : Double = 0

    var accelerationX : Double = 0
    var accelerationY : Double = 0




    override func didMove(to view: SKView) {

        for i in 0..<textureCount {
            let texture = SKTexture(imageNamed: "Ball_\(i+1)")
            textures.append(texture)
        }

        self.physicsWorld.contactDelegate = self
        self.physicsWorld.gravity = CGVector.zero
        self.physicsWorld.speed = 1.0

        self.size = CGSize(width: 320, height: 568)

        setupObjects()

        motionManager = CMMotionManager()
        motionManager.accelerometerUpdateInterval = 0.1
        motionManager.startAccelerometerUpdates(to: OperationQueue.main){data,_ in
            self.accelerationX = data!.acceleration.x
            self.accelerationY = data!.acceleration.y
        }
    }


    func setupObjects() {

        //壁
        let sprite_left = SKSpriteNode(texture: texture_left)
        let sprite_right = SKSpriteNode(texture: texture_right)

        sprite_left.position = CGPoint(x: 80, y: 284)
        sprite_right.position = CGPoint(x: 240, y: 284)

        sprite_left.physicsBody = SKPhysicsBody(texture: texture_left, size: sprite_left.size)
        sprite_left.physicsBody?.isDynamic = false
        sprite_left.physicsBody?.categoryBitMask = 1 << 0
        sprite_left.physicsBody?.contactTestBitMask = 1 << 1
        sprite_left.physicsBody?.restitution = 0.9

        sprite_right.physicsBody = SKPhysicsBody(texture: texture_right, size: sprite_right.size)
        sprite_right.physicsBody?.isDynamic = false
        sprite_right.physicsBody?.categoryBitMask = 1 << 0
        sprite_right.physicsBody?.contactTestBitMask = 1 << 1
        sprite_right.physicsBody?.restitution = 0.9

        self.addChild(sprite_left)
        self.addChild(sprite_right)

        //Ball
        for i in 0..<ballCount {
            let textureRand = Int(arc4random_uniform(textureCount))
            let ball = SKSpriteNode(texture: textures[textureRand])
            ball.size = CGSize(width: 30, height: 30)
            ball.position = CGPoint(x: self.size.width*0.5, y: self.size.height*0.5+CGFloat(i)*0.1)
            self.addChild(ball)

            ball.physicsBody = SKPhysicsBody(texture: textures[textureRand], size: CGSize(width: ball.size.width*1, height: ball.size.height*1))

            ball.physicsBody?.categoryBitMask = 1 << 1
            ball.physicsBody?.contactTestBitMask = 1 << 0 | 1 << 1
        }
    }


    override func update(_ currentTime: TimeInterval) {

        self.physicsWorld.gravity = CGVector(dx: CGFloat(self.accelerationX)*accelerationSpeed, dy: CGFloat(self.accelerationY)*accelerationSpeed)
    }


    func didBegin(_ contact: SKPhysicsContact) {
        print("Contact!")
    }
}

[GameViewController.swift]

import UIKit
import SpriteKit


class GameViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        if let view = self.view as! SKView? {

            let scene = GameScene()
            scene.size = view.frame.size
            print(scene.size)

            view.presentScene(scene)
            view.ignoresSiblingOrder = true

            view.showsFPS = true
            view.showsNodeCount = true
        }
    }
    //...
}

「Frame_right」はグリーンの縁の部分の画像です。

＜画像２＞