Hi
Thats an interesting question - I have two of those files too! I found this explanation on cyanogen mods forums
The thumbnail cache is a raw file containing thumbnail cache of images and other media, each thumbnail is 10KB. It is indexed by the id of the file in the media scanner. Which means that if you have a file with the id "30", it will skip ~300KB from the start of the file and record the thumbnail data there. But if you have a file with the id "30000", it will skip 300 MB and record the thumbnail data there. Every time a file (any type of file) is added to your sdcard, the id keeps increasing and is never reused. Clearing mediascanner data will help keep the id low, but it is not a permanent fix.
EDIT I also found this numbered point explanation on XDA forums:
1.Stock camera app indexes the thumbnails and saves the respective image's properties in a thumbdata file.
2.The problem is that each and every pic present in your memory card will be indexed in the thumbdata file and even if you remove the image, the indexed properties of the image still remains dormant in the thumb data file.
3.If the removed image is again restored in the memory card, it creates another new id for the image and indexes it once again so two entries are present for a single image (it may be the same image for us but the operating system takes the date of creation of the image in the sd card into consideration so it's another new image for it)
4.This thumbdata file increases in size in time if you keep on adding and deleting images in your sdcard because new entries gets created and old entries are not at all removed and still remains in the thumbdata file as dead weight.
5. Eventually it's size increases to hundreds of megabytes that's why you have a heavy thumbdata file even though at the particular point of time your gallery is only few megabytes because you have added and deleted files before and those entries remain there in the thumbdata files.
6. This thumbdata file can be deleted with no hesitation what so ever because it's not so important and it eventually gets generated again.
7. You may ask "what is the purpose of this thumbdata files?" the main purpose is to load images faster into memory using the indexes generated so that when you open apps like gallery it doesn't lag to display the thumbnails.
8. That's all I know . I might be wrong somewhere but that's the whole point of thumbdata files generated, getting so heavy.
9. This is again, sourced from google
It seems its not harmful to delete it but it will reappear each time you reboot.
Roger