I have been spending some time with a couple very large customers lately. Since large is relative, lets narrow that down a bit: customers with more than 50 TB of backup per night.
When you get to that size, as Chuck Hollis will tell you, scale matters. It matters for virtual tape and backup no less than it matters for standard IT infrastructure.
How do you scale? Where are the pain points? Can you keep going? What if loads doubled or trebled?
Basically: how big is big enough?