Deleted thumbnail file but no space was freed

I'm using Sony Xperia Z3 and used File Commander to browse and found that I kept running out of space because of a 4GB thumbnail/preview file/folder that grows rather quickly. I tried deleting the folder, but after deleting the folder, I saw that no space was freed and it was still there. So I tried moving the folder to my SD card which resulted in 4GB of space taken up on my SD card and no space freed. I don't think I can find the thumbnail preview folder anymore, but I think new files are building up because I'm running out of space again. Does anyone know what's going on?.