Text takes very little, but high definition scans and videos do. A very high quality scan of a single page can be in the 100 MB range, as they aren't looking just for the text itself, but tiny details in the fonts and watermarks, perhaps even in the way the paper itself is constructed. A couple million ballots would use around 100 TB. There may be multiple scans in various wavelengths. There is also other evidence, interviews, data from tests, etc.

So I can easily see a group that large generating enough evidence to need "hundreds of terabytes" of storage.

I have in the neighborhood of 100 TB of drives connected to my computer and not a whole lot of room left. I can't add a whole lot of drives to that, or I'll run out of drive letters. I don't have the money for an NAS yet. People who don't deal in big data or don't even have a real desktop computer have no idea.