Well, I'm not 100% convinced....
I had one car, a '34 coupe in which I'd rebuilt the engine, re-cored the radiator and rebuilt the pumps. Guys that know me, know I don't skimp / cut corners. That car would always push a little fluid out of the radiator after a decent drive. Removed the thermostats and no more issues.
No, I can't explain the "why", but it worked. If I didn't know who built this car, I'd suspect underlying issues.
Coolant expands when hot (meaning at operating temperature of say, 180 deg.). Without thermostats your engine never got up to operating temperature so the coolant didn't expand as much. After your car pushed out that coolant, did the level then stay the same?
How long did that engine last before showing blow-by and using oil? Cold running engines don't fully vaporize the fuel so oil is washed off the cylinders by raw gasoline, which wears the rings and cylinder walls faster.