That's a browser's (Chrome in your case) feature.
On every website (and not only on q2a) and with almost every browser the user can zoom in (increase the dimensions of the page/fonts displayed on the browser) and zoom out (decrease it).
This of course can origin weird effects on your website.
That's where responsive design (see q2a responsive themes) (and similar other approaches) comes in....
No fixed widths layout, but layout adaptable to all resolutions.