Text messaging is a hack.

Text messages aren't just tiny, they're also free riders tucked into control channels, or space reserved for operation of the wireless network. The channel uses space whether or not a text message is inserted.

This explains why a message has limited character space. It must not exceed the length of the message used for internal communication between tower and handset to set up a call.

http://www.foxnews.com/story/0,2933,473924,00.html