That is normal. Your display has a resolution of 1920x1080 and of you render less pixels it has three options:
- Render the smaller image at the Upper left corner.
If you generate an 800x600 image then just use the first 800 pixels on your 1920 display. - Do the same but center it.
- Stretch and deform the image, lowering quality.
Option #3 can usually be enabled or disabled in a laptops BIOS,
or, if you use the right drivers, it can be selected in the graphical cards settings.