The A350 carries up to 28 LD3 containers, compared to 44 on a 77W (or equivalent in pallets/containers). Of course how many of those containers are available for freight (as opposed to baggage) depends on how the airline configures its aircraft. But SQ tends to run its aircraft in a fairly low density passenger configuration, leaving plenty of space for freight.
The 77W is a superb freighter, and is responsible for seeing off dedicated freighter aircraft on all but the highest demand cargo routes.