Thanks all of you to pay attan. and give time i have reset thumbnail cache through registry as brink sir suggested its a game of thumbnail cache in my case its been disable however and so i have to reactivate it.
i have clear the thumbnail cache through registry settings.
here is a link below: Thumbnail Previews - Enable or Disable
but before applying registry key to first disable and then enable it is strongly advisable to create restore point of windows os, and take registry backup.
once thumbnail cache has enabled, the size of thumbs.db increases as you open image files.
thanks once again all of you to pay attn. and your time.