You cannot use the thumbnail to resize the image. Could you check the url that is generated after you execute the code? It should be something like this one
If you query the image directly from the browser with size=400 is there any difference.
Also clear the browser cache before trying these suggestions.
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