We've did a lot of changes to the Expander and we've introduced this problem with - not-stretching the content. The problem is now fixed and will be available for download this Monday with our Latest internal build. Also note that you can easily workaround the problem if you change the Expander's ControlTemplate as it is described here
. (Also may note that the expander style is now hugely reduced and easier to maintain).
Huge sorry for that inconvenience.
The Telerik Team.