"I miss you more" is part of a comparison. Without the other part, it doesn't have any meaning. If your mate said "I miss you." and you answered by saying, "I miss you more.", there is an unspoken, but understood part of your answer. Your answer would mean, "I miss you more than you miss me." the comparison is "than you miss me." The comparison tells how much more "I miss you". It is common to say things like: "I miss you more than ever." This means, "I miss you more than I ever have before." "I miss you more than I can say." This means, "I don't have enough words to tell you how much I miss you." I have given up eating ice cream, even though I love ice cream. If someone knew this fact and heard me say, "I miss you more than ice cream.", it would mean a lot to them.

You are absent for a long time.

it means seni çok özlüyorum