iPhone App, Storage Usage

We have been away from home for nearly 8 weeks and, when connected to Wifi, the MyCloud app on my iPhone has performed faultlessly.
However, my 16 gb phone is always complaining that the storage is almost full. When I look at what is using the space, the worst offender is the WD app.
The information says that the app itself consumes less than 50 mb, so why is the app using way over 5 gb of my phone’s storage?? I have only about 50 mb of files in the download section, so where is the rest?
Is there some way, short of deinstalling and re-installing the app (don’t want do this, I have no way to re-authorise the phone) of doing a "garbage collection)?
Really puzzled

Clear the cache of My Cloud app… Why can’t you uninstall and re-install the MC app; what does re-authorize have to do with it?