Low resolution problem after the extracting to computer

[dahaka1907]

Hi guys.

I used to have xperia Z3 like two months ago. Then I've bought xiaomi redmi 5 plus, so i decided to take my photos from Z3 to the computer in order to save them. The total size of these photos are nearly 14 GB. I did the operation then I sold my z3. After that I've realized the photos that I've in my computer are in very bad shape. They are like preview images. The resolutions are much lower than the original sizes which I've seen in my z3 device. The interesting thing is, in DCIM folder, there are nearly 4000 images and 2 thumbs files.

.thumbdata3-1763508120, .thumbdata3--1967290299

these two files have nearly the 13 GB size. and the other 4000 images have approximately 200 mb.

So I need to ask, could be my original photos in these two tumbs file? I did not delete them I've just extracted them to my computer from my android device. So where is the original sized photos?

I researched on the internet but I couldt able to open these two thumb files.

I've tried to take these files and photos to my new device (redmi5), but the photos are still in low resolution.

How can open these two thumbs file? Is that the only way to regain my originally sized photos?

Thanks for reading this.