There doesn't seem to be a direct connection between phone thickness and battery time. Nokia phones are thicker than other phones. SE has been focusing on thin phones a while (though dumbphones) and both the Milestone and Hero phones at work makes my N900 feel quite chubby.

They all have better battery time than the N900.

From looking at the N-8 prototype, it seems Nokia has had a look at these phones. But I don't like the prototype photos.