RadDropDownList has an attribute "DropdownWidth" that controls the width of the LIST when it drops down.
If you set the width to a specific number of pixels (say 200px) then there is no issue. But if you set it to "100%" then I would have expected it to be 100% of the owner controls width. But this is not the case, and it becomes very evident when the RadDropDownList is used with a template column of a RadGrid.
In a RadGrid template column, this setting causes the LIST width to expand all the way to the right edge of the screen. This seems to me to be a bug, as it makes no sense to behave this way. You can see this behavior in the screenshot I have attached.
It becomes more evident that this was not the intended functionality (and must be a bug) when you substitute the control with a RadComboBox, having exactly the same attributes. This will use the combo's default settings and show a dropdown list. With the RadCombBox the LIST does not expand to the far right of the screen, it behaves as expected, having 100% of the combobox width.
I would expect both controls to work identically with their LISTS when setting the "DropdownWidth" attribute to "100%".