One-to-one vs. many-to-many messaging. The amount of work you need to do to deliver a WhatsApp message is constant and small -- just route the message to a single recipient's mailbox. The amount of work Twitter has to do to deliver a message grows as a function of followers. One celebrity tweeting another celebrity means you have to deliver the message to the mailboxes of the followers of both -- millions of times more work than WhatsApp per message. In addition, Twitter persists all the messages while WhatsApp doesn't.