MacBook Pro internal display resolution wrong

I've got an early 2013 MacBook Pro with an external display connected via HDMI to the in-built port. The system shows that the resolution of built-in display (15-inch Retina) is 2880 x 1800, while the external display max reso is 1920 x 1080. So if I have a large picture / application on the external display, I should be able to drag it to the internal one and have still screen estate to spare. This isn't happening. Here are images of this very page, first on the external screen, then without resizing dragged to the internal one:

I can't change the resolution of either display in Display Preferences. OS is Mojave 10.4.5, and as far as I am aware everything is up to date. Can anyone suggest what's wrong, and what could I do to fix this?

This is the correct behavior. The actual resolution of the panel is 2880 x 1800, but macOS scales the UI to simulate 1440 x 900 @ 2x by default.

You can change the scaling factor in Display preferences to "create" more screen space. If you set it to the highest setting, 1920 x 1200, then you'll be able to drag a window from the external to the internal display in the way you're expecting.