I'm not quite understanding.
Both are the same picture, except one is upscaled compared to the other.

If you're trying to make your client full-screen, You may do so by clicking http://image.prntscr.com/image/4268ba757201487fbf4be55205caa332.png

However, if that doesn't help. It's more of a PC issue where you'll have to adjust your screen resolution.

Adjust your resolution and check.
Regardless you can manually adjust the client size by gradually hovering over the edges and dragging it.