When I try to salvage another player's wreck it says: You cannot salvage the wreck because it contains too much loot to fit into a single container. etc...
it could be you dont have enough room in your cargo space, ideally you need at least 600 m3 of room, i tend to follow up missions with a ab fitted mammoth, tractor/salvager and over 4k of cargo space, a decent level 4 mission usually gives you at least 1k m3 of salvage/loot. .. so if your trying to loot a ship in a frigate, then it might just be too small