I had a lot of memory problems with Sprite Kit, and I used a tech support ticket to get info, and it may relate here. I was asking whether starting a new SKScene would totally release all the memory the previous one used. I found out this:
The underlying memory allocated by +textureWithImageNamed: may or may not (typically not) be released when switching to new SKScene. You cannot rely on that. iOS releases the memory cached by +textureWithImageNamed: or +imageNamed: when it sees fit, for instance when it detects a low-memory condition.
If you want the memory released as soon as you’re done with the textures, you must avoid using +textureWithImageNamed:/+imageNamed:. An alternative to create SKTextures is to: first create UIImages with +imageWithContentsOfFile:, and then create SKTextures from the resulting UIImage objects by calling SKTexture/+textureWithImage:(UIImage*).
I don't know if this helps here.