Do you have RadGrid's height property set? If not try setting it to 100% - this way RadGrid will occupy the whole RadPane and will resize along with it.
If this is not the case, could you open a support ticket and provide a sample runnable project so we can investigate it further?
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items.