The problem is that, in 2dbar, the width of the actual graph is tied to the width of the chart background. The 3dbar's x axis is not tied to the chart's width, so when you change mschart.width, it adjusts the chart's background width and just moves the 3dbar graph to the new center. I did see that if you change the Chart's height, that it will make the 3dbar graph larger though.
The best solution would be to change each row's width without altering the size of the graph backgound at all, but I don't see a property for that either.