The dirt on the forehead of a pig makes a small mound. Picture it, if you will. A pig is rooting through the dirt. It lifts its head and there is a mound of dirt right on its forehead.
The pig's forehead dirt mound keeps getting bigger until it's the size of two cars (つか)! Pigs are big, but you didn't think one could balance a mound the size of two cars on its forehead and not fall over.