padx values seem to affect the arrangement of tkinter Frames
The reason is that all three frames are in a single column. Since a column has uniform width in a grid, if you make the top frame wider it makes the whole column wider, and that affects how the second and third frame appear.
My advice would be to use pack for the three top-level frames. I would pack MenuBar along the top, StBar along the bottom, and then MidSpace in the middle.