Yes, I believe this is controlled by either the "Virtual" or the "ViewPort" tags in your XF86Config file. Some people prefer to have a 1024x768 virtual screen when their display may only be capable of 800x600. You would only be able to see 800x600 at a time and be able to scroll to the rest of the 1024x768 desktop.
I personally do not like this behavior either. If you find a "Virtual" tag in your XF86Config, comment it out and see if that fixes it. See "man XF86Config" for more info on those tags. I do not have either of those tags in my current XF86Config file so I believe if they do not exist you will not have a "virtual" sized display. If it does not exist set it to "Virtual 1024 768" if that's the size you want. I have configured it both ways in the past.