Hi Nebojsa Danilovic,
Please accept our apologies for the caused inconvenience. During the last Q the Expander control went under extensive changes. Our goal was to improve the performance of the control and fix many of the bugs. Unfortunately we have introduced an issue related to the Alignment properties, which is already fixed and you should be able to download it with our next internal build.
In the meantime I attached a sample project where the default style of the RadExpander is modified to allow stretching. Please give it a try and let us know if it works for you.
All the best,
the Telerik team