RadChart is designed so that the bar width actually depends on the number of items. The more the items - the narrower the bars. This provides the necessary level of flexibility, so that the chart can handle multiple cases with minimum adjustments. Still, in cases like this one RadChart allows you to fix the bar width by fixing the XAxis range. You can use these two lines:
RadChart1.PlotArea.XAxis.AddRange(0, max, 1);
is defined by the max number of items in the underlying datasource. This way the X axis will always reserve space for max
items, and even if there are less items in the chart the bars will not grow.
All the best,
the Telerik team