One tweet is more like 670 KB, https://news.ycombinator.com/item?id=3667072 Most numbers in that article are way off.

That's still waaay too much for a 140 character message and a thumbnail of the author, plus possible custom css.

created_at, hashtags, user_mentions, favorited, geo, id, retweet_count, retweeted, source, text, truncated.

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)

Small correction: the first twitter page downloads ~2MB. Most of the images, JS, and CSS get cached, so every page after the first downloads much less data.