Hi Qu Luu,
Most probably you are loading the TreeView in expanded mode (i.e. some of the items outside the view port are expanded). This could be the reason for changing the scroll thumb size while scrolling.
If this is the case you could collapse these items and let the user expand them explicitly. If you would like to preserve some user state you could use the BringPathIntoView method to show the required node.
Unfortunately there is no out of the box way to manipulate the thumb size and stop it from shrinking.
If the suggested approach does not fit your needs, could you please share a bit more info about you application and what are you trying to do?
All the best,
the Telerik team