Not sure what you mean by “these sizes”, but no size is fixed in Xtreme, so that the site is responsive and will adapt to any device / screen resolution. You can see this by reducing your browser width and observe what happens.
When you set a theme width, this is actually a maximum width, which mean the site wont be larger than the set width. But when the browser gets smaller, everything will scale and reorganize automatically.
Of course the theme has no control over your content, so it is important that when you create page content, you follow responsive design rules, and in most cases this means not use fixed width styling, but either % or max-width.
If you have more questions, open a separate thread with a link to the site