Sankey Boxes size not mathcing

Hello 

I am trying to create a sankey daigram, but I seem to have a problem with the box sizes. As you can see in my example, box "c" should have the same size as the arrow coming out of box "a" to box "c", since the sum of arrows going out of box "c" is eqaul to the amount coming out of box "a" to box "c".

Thank you for the help.