It's still swollen from when she was dropped on her head as soon as she came out of the womb, which resulted in her stupidity, uselessness, retardation, and sickly pre-disposed obsession of the Sauce.
I never understood why they said it was big... Everyone else's is almost or just as big. She just doesn't wear the head band over her fore head like everyone else. I know Hinata has a bigger head too, she just covers it with her bangs.
sakura's forehead is no bigger than tsunade's,temari's or anyone who doesnt have hair or a forehead band thingy covering it,y'all shud just stop being so pretentious