well i mean you said it has "gotten bigger", and that usually means its not a visual glitch, and it has nothing to do with cracks, seems like this was a bad screen to begin with, AFAIK these types of problems are usually caused by layers in the screen being defective, i dont know, although that is only in the case of it being a physical issue, it could be a graphical one, try reseting your iphone, and if the spot is still there WHILE its booting, it could be a screen that is failing