When used correctly, XHTML is lighter than HTML.
It's suggested to not use table anymore with XHTML for the global display of your site. You should use <div> for blocks(like a cell with HTML), lists(<ul> and <li>) for menus, the headers(h1, h2, h3...) for title and so on...
Of course, XHTML is maybe harder to understand than HTML, but when we know all its advantages, XHTML is better.
Just as an exemple, see at the code of my design here: http://sadike.free.fr/sadike/ftxp/index.html . If you want, try to do the same thing just with HTML and tables.... I'm sure that you couldn't do lighter!