Why is he talking about 'terabytes of source data' then? If they're procedurally generated, then stored and rendered, they "are that large". Maybe I'm missing

Why is he talking about 'terabytes of source data' then? If they're procedurally generated, then stored and rendered, they "are that large". Maybe I'm missing something. [edit: also he talks about the workflow the artists use -- they are working with large images as well -- doesn't sound procedural, particularly]