The same information will be shown when user hover a record in the Recent Items list on the sidebar.
Can we modify fields show in the popup? YES, example we want to customize field in Account hover when user hover it from Contact page layout:
- Don't be confuse that we are not change anything in Contact Page Layout, although hover happened in Contact page
- Go to Account object and edit Account Page Layout assigned to particular profile and record type (if any)
- Click Mini Page Layout at the top of the editor
- Select fields you want it show when user hover over the object
- There are field that you cannot remove, such as: object name, Parent Account for account object (to hide this, you can use Field Level Security, but of course field will be not visible anywhere, from: page layout, report, and etc)
- There are field that always shown, example in User object, it will always show: Title, Email, Phone, Manager