You're correct. And to complete it:
"Larger content (Concatenated SMS, multipart or segmented SMS or "long sms") can be sent using multiple messages, in which case each message will start with a user data header (UDH) containing segmentation information. Since UDH is inside the payload, the number of characters per segment is lower: 153 for 7-bit encoding, 134 for 8-bit encoding and 67 for 16-bit encoding." -- from Wikipedia
So, in this case it's 134 bytes and not 140 since the payload probably doesn't fit in a single 140 bytes.