The SMS limit was decided when an engineer working for German Telecom decided on the limit after typing up a bunch of sample sentences. Twitter based their limit on the SMS limit (140 + user address).
> decided on the limit after typing up a bunch of sample sentences
From what I heard, the SMS limit was actually a protocol limitation: it piggybacks on signaling messages, which have a small size limit (this is also why SMS can sometimes work even when everything else doesn't). Quoting from Wikipedia (https://en.wikipedia.org/wiki/SMS):
"Messages are sent with the MAP MO- and MT-ForwardSM operations, whose payload length is limited by the constraints of the signaling protocol to precisely 140 bytes (140 bytes * 8 bits / byte = 1120 bits). Short messages can be encoded using a variety of alphabets: the default GSM 7-bit alphabet, the 8-bit data alphabet, and the 16-bit UCS-2 alphabet. Depending on which alphabet the subscriber has configured in the handset, this leads to the maximum individual short message sizes of 160 7-bit characters, 140 8-bit characters, or 70 16-bit characters."
That's not what is meant. Twitter was originally an SMS service, which would text you whenever someone you followed posted a tweet. In order for you to be able to tell who had posted each tweet, the message had to contain the poster's username. This is distinct from the username of whoever they were replying to.
From what I heard, the SMS limit was actually a protocol limitation: it piggybacks on signaling messages, which have a small size limit (this is also why SMS can sometimes work even when everything else doesn't). Quoting from Wikipedia (https://en.wikipedia.org/wiki/SMS):
"Messages are sent with the MAP MO- and MT-ForwardSM operations, whose payload length is limited by the constraints of the signaling protocol to precisely 140 bytes (140 bytes * 8 bits / byte = 1120 bits). Short messages can be encoded using a variety of alphabets: the default GSM 7-bit alphabet, the 8-bit data alphabet, and the 16-bit UCS-2 alphabet. Depending on which alphabet the subscriber has configured in the handset, this leads to the maximum individual short message sizes of 160 7-bit characters, 140 8-bit characters, or 70 16-bit characters."