The diameter of the post is a factor in the sound of the instrument, also for structural integrity the post should be somewhat proportionate to the instruments size. In a typical 3/4 I look for a 17.5mm give or take. For a larger bodied bass like the Hawkes we use a 18.5mm post, it just sounds better and offers the additional support I feel the larger top may need. This same methodology is typical with a Montagnana cello, bigger cello bigger soundpost..... Some instruments react more to this type of adjustment than others.
The soundpost gives/acts as a still or nodal point on the top, it's position and size would directly effect the contribution of this "dead" spot and how much the top is or is not responding at the given frequencies.
Again, this is just a piece of the puzzle when finding a sound, don't expect a 1mm difference to change the world. Most soundposts are poorly fitted and are not working right, I would advise first to worry about position and fit than diameter.
AND....CB why do you ask?
