How so ? Don't we all agree magic makes the body weaker ? That was quoted from wowpedia.
Also, Third Gens are waaay weaker than Second generation DKs. While we don't have a valid reason for that, common sense says it's because Third Gens are resurrected corpses, while Second Gens are still somewhat living, having joined the Lich King in life.
Which sustains my suggestion that death makes them weaker than if they were recruited while still living .