Have you ever tried
du -csh .* to find the size of all the hidden files in your home directory? It never works properly for me as I think it only counts the size of the .* files in each of the named directories.
Anyway, the following solves that problem and shows you where all those thumbnails have been generataed by gthumb or nautilaus.. 🙂
for i in `ls -A`; do du -sh $i; done