There is no real advantage to a larger outlet other than maybe the water has less force when it leaves the return. Your going to be flowing the same amount of water thru the plumbing so the larger diamater would have less force (the GPH
would be the same) than if it was comming out of a smaller diamater return.
If you wanted to upsize the return just get a 3/4 - 1" adaptor at the hardware store. They have many to fit just about any plumbing combo. Threaded female - threaded male, threaded male - threaded female, threaded female - threaded female, etc.