That's still waaay too much for a 140 character message and a thumbnail of the author, plus possible custom css.
That's just a small selection of the important elements delivered in the single tweet JSON packet. I didn't even get into the 'user' stuff, which is included so that all of the other fun things about the user can be displayed.
(Note: this is using the web interface to Twitter, which is what I'm assuming the OP was implying)