Probably just anecdotal numbers from experience. I've seen similar things as well. The division of prospects/accounts is kind of a big deal, and usually defaults to geographical area or industry as the primary dimension, with size/revenue of prospect as the secondary.
Given that, if you're selling something like Github or container management services, you can imagine that the person who's selling in the Pacific West Coast region might be having an easier time of things than the guy who has the mid-west. This isn't universally true for all products, but sales people will complain about leads being bad in any industry (see: Glengarry Glen Ross).
All of that is to say is that empirically, usually there are a few outperformers on any sales team that are crushing quota, and a bunch of others who are skating by at fractions of quota until they get canned. As some evidence, there's a lot of churn in sales teams and for the average sales person, which is partially because of the above, and also because good sales people will kill it as a company is getting a lot of adoption, and then jump ship when a better opportunity arises.
So maybe there's been a study on this stuff, but you could talk to any number of people who work in sales orgs and hear similar stories without the study.
Oh, and yes, when companies get 'serious' about sales, you'll see sales quickly expand to encompass a third or half the total company size. I won't make any guesses as to whether that actually makes sense and how many people are being productive, but large sales orgs are common in my understanding.