You are seeing black boarders because the MacBook display resolution is lower than the iMac display resolution.
You can set the iMac's display to a lower resolution. This will make the window containing the MacBook display appear larger, but does not change the resolution of MacBook display shown in the window on the iMac. Turning scaling on or off has no effect.
If the MacBook display resolution can be set higher than the iMac display resolution and scaling is turn on, then the MacBook display can appear on the iMac at a lower resolution than current set on the MacBook.
You can also choose to use any available VNC viewer. I tried Chicken and found this also did not allow scaling when the MacBook display is set at a lower resolution than the iMac display. However, I did find that RealVNC VNC viewer does allow full screen mode with scaling to remove the black boarders. Both Chicken and RealVNC VNC Viewer are free applications.
Note: This answer was tested using a 21.5" 2011 iMac and 21.5" 2013 iMac connected by Thunderbolt. I do not actually have a MacBook.