HemantPatel, I don't know if you're still needing an answer to this as your post is old. But, just in case. I had the same issue with a stupidly tall vertical column header row. I posted my issue after searching the community and not finding the answer, I did come across yours though.
I was provided an answer by the legend known as gwassenaar, it turns out the issue was that I had not provided a label for the expression and so the height of the box became the length of the expression formula. I set the label to =' ' (single space) and this cured the problem.