Its Nothing,But A Copied Files Of All The Images Available In Sd Card...And Its Size Too Increases...
Myn Highest 20mb ..
How Can We Stop That ??
Haha... mine is 110 mb :P
the .thumbnails folder actually contains a small sized copy of all the images you have on your sd card. It is created by the Gallery3d.apk so that whenever you open the gallery you can see a small sized preview of your images without opening them.
You really can't get rid of it unless you delete the Gallery3d.apk from system/app