Is it normal for cache file to be too big?

I'm of the opposite opinion concerning the 'World Cache' files. I do delete these files once they reach a certain huge file size. It hasn't hurt my game. However, the loading of textures and other things happen a bit slower on the next start up until the file is rebuilt. If you lack patience or if seeing those grey texture-less objects bothers you, don't delete these files. Once everything has been 're-written/re-loaded', there is no need to 'clean it' again until it reaches that certain huge file size. What that huge file size would be depends entirely on you and your machine.

Deleting the thumbnail package is a big no-no for me as it can play havoc with the images of installed content, even content which is hardwired into the game and not store bought or otherwise installed. I leave this one alone at all costs.