whats the upside and downside of having a larger diameter rope?

Beefier ropes are more durable and offer greater friction when running through carabiners and belay/rappel devices. This means you will have a rope that will last longer and and will be able to control the rate of descent better when rappelling or lowering someone off of a climb. For top roping/ repealing most climbers will use a rope 10+ mm.

Skinnier ropes provide less rope drag, combined with being generally lighter, a lead climber will exert less energy using a skinner rope. But the skinner ropes, generally, sacrifices durability and will have less friction available when belaying/ repealing. Most lead climbers will use a 9.8+ for everyday use, and save there 9.8- for red point burns and hard on sight attempts.

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