That could be because you have kept the width of your table/div box fixed i.e. defined the actual pixels your box would be covering. For the contents to be spread out at a higher resolution than that you have worked upon, try giving a relative width to your table/div.
Something like <table width="80%"> </table> will do. The width of your table will then be 80% of the width of the page, be the page in 1024x768 or 800x600.
