it is what it is...
In this day and age, the absolute last thing that any given developer of any given software for any random OS worries about is it's storage space consumption.
You're using Radarr correctly and for what it's designed for, but what you want is a feature req (or a larger cache drive), but if you want its behaviour to change, best to raise an issue on radarr's own project page (or alternatively delete the MediaCover folder on a schedule)