I hate to say it, but I have seen that symptom on more than one car and it always turns out to be the head gasket.

"Too much coolant" isn't really physically possible. It would puke it out of the overflow after the system pressurized but then everything would be fine. It wouldn't cause coolant to leak from other places.

Do the hoses get very hard/swollen? - this can be an indicator of combustion gas leaking into the coolant passageways
Does the system pressurize like this even when the car is not warmed up yet? - same as above
Does the pressure go away if you let the car sit overnight? - if not, it can be indicative of head gasket or head issues