For e.g .Due to rusting , iron becomes weak .So it should become weak only if the amount of iron is reduced. Even if we say the reduced iron is now the rust ,the amount of rust practically is very less comparing to iron.
In chemistry we assume the total mass of reactants to be conserved in after reaction (except in the nuclear reactions, of course). It is a fundamental law and hold true everywhere.
Even in your question the Oxide layer formed above the iron makes the Ferrous Oxide which generally get removed, hence you think the mass had reduced.