You're forgetting Amazon is a "marketplace" and not just a "retail store."

Anyway, I don't think "markup" is very much related to how strong they are. I define "strong" as "how critical for business that service is."

I'm just using Amazon as an example, one that everyone can understand has a massive amount of power.

I think of distributors, not retail stores, to be pretty much the strongest middlemen there is. If you're making a widget are you going to hire someone full time to market your product to individual stores and then hire and maintain a fleet to deliver them? Fuck no you aren't, you're going to call a distributor.