[development] question on page layout

I know this is a very basic question and this may not be the appropriate forum, but: Often when I browse Drupal sites (or really any CMS site), the page is just a bit too wide for my browser and I have to scroll left and right a tad. It actually looks as though it is going to fit but it then widens and does not. Also, many times there is a very long blank space extending downward, such that it makes the page appear empty to the casual viewer. You have to scroll down to find content, apart from menus that may be on the left or right. Are these things preventable? Or, are they to be expected when dealing with CMS tools? I appreciate your time. Josh