The phrase "big foreheads" is often used to describe people who have a noticeable or prominent forehead. It is just a way of referring to someone's physical appearance.
Sentences with «big foreheads»
It is a especially designed hairstyle for men with big foreheads. (menwithstyles.com)
Big forehead smack that I didn't put some bacon in the salad in the first place. (strawberriesforsupper.com)
The best suitable hairstyles for big foreheads are the really casual and cool ones. (menwithstyles.com)