Ask HN: How does twitter allow multi-byte messages

I posted a test message on twitter to check if it accept multi-bye chars and posted "信" 140 times, which it accepted.

How does this work? Looking up wikipedia, I realized that GSM accepts 7-bit characters, and for multi-byte it uses USC2 (UTF16)

Wouldn't this message be double the size in this case(70 16-bit characters)?