With the launch of the next Xbox dashboard update will come a slight change to the proportions of the Avatars. These changes were made to allow better mapping between players with using Kinect to control their avatars. Existing avatar clothing and other assets have been updated to work properly with these new proportions.
For games that include support for Avatars, read the following thread in our Forums to understand how these changes affect games made with XNA Game Studio 3.1 and 4.0, and the changes that you can expect when XNA Game Studio Connect for 4.0 goes final.
Forums: Update to Avatar Proportions in XNA Game Studio 4.0
We encourage you to post any questions you may have directly to the thread; we want to make sure this change goes smoothly for everyone's games.