That looks like you have probably Update Accelerator enabled and those files will likely be the caches for your downloaded updates.
You can adjust the maximum disk usage on the Update Accelerator menu page (under top menu item Network).
If you click on the Statistics button it will also show you the information about the caches and how much the cache is consuming.
It will also give you a list of all downloads that have been cached and you can purge specific downloads or you can purge everything from that page that has not been accessed for more than some period of time.
The only other thing I can think of it being related to is the Web Proxy cache. If it is related to that then you can adjust the cache size on the hard disk on the Web Proxy menu page (also under Network menu)
Quick look through the wiki page on the Update Accelerator and that confirms that directories under updatecache are created by Update Accelerator with a directory per vendor. Inside each directory are all the files that have been downloaded and cached.