Hardly huge. When I was at a web agency with about 70 people on staff here in Australia, we were an AWS partner with negotiated rates, and honestly we weren't doing massive clients really (aside from some government stuff now and then).
It has happened, though I'm not sure if it's common or not. I worked for a big hadoop user (5 years ago: 6k boxes/over 50k cores/30 pb) that aws really wanted on their platform. They offered huge discounts to get us as a marquee customer, much better than their public pricing. Unfortunately, they were still ludicrously expensive compared to O&O but we were good at running huge automated clusters with 1.5 ops.
TIL. After reading about Spotify negotiating with Google Compute Cloud I just thought it was common place for such orgs to negotiate vs everyone else that will rarely reach that scale.