I have HTC One X running Android 4.2.2. While typing SMSs if I put an emoji character then it sets the maximum number of characters to 70. Why?
Because when you use emojis, you force the use of UTF-16 encoding instead of the regular 7 bit ASCII encoding. 7 bit ASCII encoding allows 160 characters per SMS, but since UTF-16 encoding takes up about twice as much data per character, it only allows 70 characters per SMS.