Can you provide more details? What version of XD are you using? When you use the centering option the page is centered in the browser. If the page is smaller than the available browser width or height there is space on the sides.
The new version of Web Export there is a new Scale to width option. This will scale the artboard up or down to fit all available space.
It’s available now. Select the artboard and in the Export Artboard dialog set the Scale option to Scale to width and check Scale on resize. You may also need to set the horizontal and vertical positioning. Resize the browser and test to get the behavior you want.