The shared thumbnail folder can be quite sizeable on linux (or whatever OSs follow the
freedesktop.org specifications), you won't have much trouble finding forum threads of people asking if it's OK to delete it, because it's occupying too much space.
One of the reasons it occupies so much space is because the thumbnails are set to be PNG files, which are comparatively large by nature. Thumbnails for a folder full of JPEGs of moderately medium-large resolution (about 800-1200x800-1200)...
