"Why" that mode is chosen, I can't say, but the first thing you will want to do is check the available modes...
DisplayMode modes[] = GraphicsEnvironment.getLocalGraphicsEnvironment().getDefaultScreenDevice().getDisplayModes();
for (DisplayMode mode : modes) {
System.out.println(mode);
}
So, on my 2017 MacBook Pro, running macOS Maojave (10.14) I get...
2880x1800x32bpp@[Unknown refresh rate]
1440x900x32bpp@[Unknown refresh rate]
3360x2100x32bpp@[Unknown refresh rate]
2560x1600x32bpp@[Unknown refresh rate]
2048x1280x32bpp@[Unknown refresh rate]
1650x1050x32bpp@[Unknown refresh rate]
1280x800x32bpp@[Unknown refresh rate]
1152x720x32bpp@[Unknown refresh rate]
1024x768x32bpp@[Unknown refresh rate]
840x524x32bpp@[Unknown refresh rate]
800x600x32bpp@[Unknown refresh rate]
640x480x32bpp@[Unknown refresh rate]
1680x1050x32bpp@[Unknown refresh rate]
But, as you can see, 1080p isn't supported here :(, so if I was in your shoes, I'd need to find something which was close. Maybe, 1650x1050
or 2048x1280
How does that help you? Well, once you've determined the best mode for your needs, and according to the documentation, you can simply do...
GraphicsDevice device = GraphicsEnvironment.getLocalGraphicsEnvironment().getDefaultScreenDevice();
device.setFullScreenWindow(frame);
device.setDisplayMode(mode);