Without knowing the model number/name it's hard to determine exactly what the specifications are. Have a look at this article: https://www.intel.com.au/content/www/au/en/support/articles/000023781/graphics.html
If the two monitors you're trying to use have different resolutions then you'll be stuck at the resolution of the lowest common denominator. eg: if one has 1280x1024 and the other 1920x1200 they'll both display as 1280x1024. And bear in mind the different connection types (HDMI/VGA) have different maximum resolutions too (see the article above).
My best guess would be that the monitor(s) has a higher resolution than the laptop screen so it's being limited to the lower resolution. Which results in you getting black bars. You can try following the suggestions here: https://www.intel.com.au/content/www/au/en/support/articles/000023765/graphics.html
edit: Meant to add - what version of windows are you using? In windows 10 display settings there is an option relating to multiple displays and you can pick duplicate or extend (you should pick extend).