This is not a known issue and it was not reproduced in our testing scenarios.
If the column width is bigger than the Grid and a horizontal scrollbar will be displayed to scroll the content. If it is not scrollable then the column widths will be ignored.
I can assume that this is caused by a custom CSS rules which are affecting the Grid appearance.
I understand that the Grid implementation is complex, but if additional assistance is needed, we will need a runnable example in order to investigate what may be causing the undesired result.
Telerik by Progress