Hi @skhanna ,
Currently, it is not possible to put column subtotal as first column in Matrix, an idea about this issue has been submitted in the following link, please vote it up.
Microsoft Idea (powerbi.com)
As a workaround, you can adjust the size of the matrix, or the size of the font to make the scroll bar disappear.
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.