why is my boiler filling with water when not in use. (gas fired,steam heat?

why is my boiler filling with water when not in use. (gas fired,steam heat ).
Endless buckets must be emptied from the boiler before the level in the sight glass goes down. Water valve is shut tight.
I don't understand the mechanics of the system, but this seems wrong. I we had turned on the heat with it so full, would we not have a flooding situation from the radiators?