There could be many reasons for this lost disk space. For example, if you have Windows System Restore activated, Windows automatically saves files so they could be restored later. Most of the times when you are uninstalling Windows applications, System restore is backing-up your files. Periodically, these files are deleted by Windows. You can force deleting these files (also called restore points), to gain the disk space back if you want.
Other reason for lost disk space is because the browsers are caching web-items.
We don't have any issues reported so far with uninstalling.
the Telerik team