What's the connection between the User Profile Service and the People And Groups hidden list? SharePoint 2013

I'm looking to understand the connection between the User Profile Service and the hidden User Information List.

Here's what I'm experiencing.. New SharePoint 2013 farm with a migrated UPS from a SharePoint 2010 environment. The UPS is functioning properly. When I create a new site collection, I expect that the hidden list to be empty except for the first few accounts that touch it. However I've got every group and user from my AD environment in there.

I thought I understood the hidden list to only retain people who have accessed the site. How did the entire AD get in there?

Thanks. Doug