Tablix cells width issue in SSRS
As it looked ok in design mode and the issue was happening at run time, it would be helpful to investigate what elements displayed in the Tablix can grow or expand at run time.

I resolved this issue by following the steps below:

1- There was an image in one of the tablixe cells, I checked to see what is the selected option for displaying the image, if its size is bigger than the cell's size. The image had set to accept the growth, so it was increasing the width of the cell and consequently the Tablix size.

2- The padding for all cells of the row needs to be checked, especially it can be simply overlooked when an image has been assigned to a cell in the Tablix. If there is a padding, it would affect the cell size and eventually the row size in the Tablix.

I hope this description helps anyone else experience this issue.