Select Start > Settings > System > Display, and look at the section that shows your displays.
You can resolve this issue by temporarily changing the screen resolution of the external display.
When the screen resolution of the built-in display is larger than the external, then the wallpaper on the external display might appear to be too big and will be cut off on the right and bottom sides. When the screen resolution of the built-in display is smaller than the external, then the wallpaper on the external display might appear to be too small and doesn't fill the whole screen. This problem might occur on Windows 10 or Windows 11 if the screen resolution of the built-in display differs from the screen resolution of the external display. On a Windows 10 or Windows 11 device with a built-in display, when you connect an external monitor with a USB-C, DisplayPort, or HDMI connection, the wallpaper on the external monitor might not correctly fill the screen.