On Dropbox I see no difference.
Downloaded to my computer and opened with Windows 7 "Windows Photo Viewer" the only difference I see is in the size of the images displayed, which is to be expected.
Not sure what you think you will get by resizing an image that is 6037 x 4025 pixels, 24 MP, down to 1240 x 827, 1.025 MP.
It seems that it lost detail, sharpness, even colour. Could you explain to me if I downsize it to a smaller image am I supposed to loose detail only when I try to bring it back to a higher size? But not when I making it smaller? Compression not supposed to reduce quality unless I would set it to do so?
When I VIEW those 2 images I don't see any loss in sharpness or detail except the reduced images has many Less pixels. If you try to view it the same size as the full images it will get pixelated.
Again I don't understand what you are thinking should happen. The reduced image lost more that 4000 x 3000 pixels in the reduction process.
You are NOT compressing, pre say, the image, file size alone, you are REMOVING pixels from the image.
If you are looking for a smaller File Size in Mega Bytes (MB) then Reduce the Quality slider and leave the pixel dimensions the same.
Again on Drop Box those 2 images look EXACTLY the same on my computer.
The 2 images created from using the Snipping tool in Windows as they were displayed by Dropbox from the links you posted where placed into one Photoshop document.
Maybe it's me but I don't see any difference.
If you try to ZOOM In sure there will be a difference. the Reduced image has many less pixels.