A hdmi2vga adapter transforms digital video data to an analog signal this process needs calculation and is not as easy to do than hdmi2dvi for example. The hdmi input recognizes your adapter as hdmi display in 16:9 than your adapter calculates a analog 16:9 signal witch is send to your 4:3 display so its absolutely normal that you see black bars. Try to set you display output from your NUC to 4:3 if possible. But what you are trying to do is really not a good idea. I bet your display also supports DVI so use hdmi2dvi. If not your display is too old get a new one.