Since it's a pivot table, and you have dragged and dropped month dimension on top of your expression, it is going to adjust all the fields which you change one. So no cool answer for you. Sorry!
Now, if you upload you app, someone may provide other tricks to achieve it.
You have your expression in a row and the months dimenstion are in column which has a seperate expression. If my understanding is correct, you can drag the month dimension to the top of the column list and by this way it will be treated as a seperate column and you will be able to change the width of the other field without affecting the month members. Refer the attached file.
Attach the QVW file if you still cannot do it and I'll try changing the table.
