Physics . . The frickin' class none of us gearheads needed when we were in high school . . .
You put 3 gallons of water in a water heater (your engine) at 50°, then heat it up to about 180° and can't understand why it pukes . . Simply because that 3 gallons takes up more room at 180° than it does at room temperature!
Your car will tell you how much coolant it has to have. If it pukes 16 ounces then it will take 16 ounces to fill it up so it'll puke 16 ounces again. It's trying to tell you something . . . It's frickin' full when it's 16 ounces low at garage temp.
Want to know EXACTLY how much water a system will take? Heat the water to 180° BEFORE you put it in . . .
Just went through this with my Boss . . . (And no telling how many street rodders)
And, here's an old Sprint Car Crew Chief trick on finding a possible head gasket leak. Fill your radiator to the top, shake the car to get any air bubbles out, and add water if necessary. Then bring the first cylinder up on the compression stroke with a long ratchet or breaker bar, count to ten and if there isn't an air bubble, proceed to the next cylinder . .