There are more rules involved here and the Airline can make a judgement call here which ever is more suitable for them.
Here at EHAM we have a limit that one person is only allowed to lift a bag of 23kg, if a bag is heavier it needs to be lifted by two persons thus increasing the processing cost for the airline. For aircraft the most important parameter is weight, more weight equals more fuel or, in extreme cases, less range. (range is traded for added payload)
So depending on the load factors of the aircraft and the amount of additional cargo the airline transports it can determine if it has the space available to transport more voluminous luggage by allowing multiple pieces that add up to 23kg per pax for example.
So there is no definitive answer possible to your question as there is a trade off to be made by the airline, filling the entire cargo hold with a high density material like lead is not possible, the aircraft would be too heavy.