I'm using Lubuntu 12.10 with PCManFm, the default file manager.
/home/myhomefoldername/.thumbnails/normal is populated with more than 2000 itens, all of them are .png thumbnails made from files such as images (png, jpg or any other image format) and from pdf files. I can't see thumbnails made of other file formats such as .odt or .ods; .mov or even made from .txt.
When are they created? From what I'm observing here, they're created when I goto folders with those kind of files. Note that I view files on PCManFM as a detailed list, not as thumbnails. You don't need to open the files I listed at the previous paragraph. Just open a folder/directory with those kind of files and the thumbnails will be created at /home/myhomefoldername/.thumbnails/normal.
Note that all thumbnails seem to have a size limit: xx by 128 pixels. On PCManFm>Edit>Preferences>Show (not sure if it's the accurate translation once my Lubuntu is not in English) you'll see options to enable/disable thumbnails creation.
Default is to create thumbnails sized 128x128 and to not create thumbnails of files larger than 2048kb. This last option is strange because I have .ods and .txt files sized 104kb and I see no thumbnails of them at /home/myhomefoldername/.thumbnails/normal.
/home/myhomefoldername/.thumbnails/large is kind of empty. There's a subfolder called 00000000000000000000000000000000.png with no files at all inside.
I hope I helped a little with your investigation.
Good luck finding more about it!