All templates apart from modern currently use Tables, which is not valid HTML for presentation for this reason - on some Joomla templates, the width just isn't right.
We're working on converting and updated all themes to be a little more nicer looking and also fix bugs, but I havn't done too much so I can't give much advice yet
Try setting a manual width parameter on the <td> or <table> tag in the index.html for the template. I had to do a similar thing with a template i'm making, but it was a div tag so i did it through CSS - dunno if width CSS works on Tables too well.