Look at the size of a modern phone compared to the size of AAA's or AA's, the phones are now smaller than the batteries.
I bought a phone with the battery on the back and removable and then bought a spare pack cheap from ebay and swap the two once a month so I get more life out of them.
BTW they are Lithium Ion not Lithium <img src="/images/graemlins/wink.gif" alt="" />