Appropriate UIImage size on iPhone 6

I got two PNG files:

• 568x320.png


• 667x375.png

If I'm creating UIImageView:

UIImageView *iv = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"568x320.png"]];

and run this in iOS Simulator > iPhone 5/5s/6 (iOS 8.1) image covers full screen (even on iPhone 6). On the other hand if I'm creating UIImageView:

UIImageView *iv = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"667x375.png"]];

and run above in iOS Simulator > iPhone 6 (iOS 8.1) image is bigger than screen. What's wrong?

PS. I'm not using Storyboard (everything programatically)

PS2. No constraints were added.