THe RESOLUTION changed !!!!!
You are viewing it in terms of visual quality and trying to find any higher detail like reflections or beautifuls lights right. That is not resolution dependent . there are other options for that .
What you have lowered is visual clarity in terms of the image produced . earlier your system was generating 1920*1080 pixels on screen , which is your res. and now it is producing only 1366*768 pixels which is roughly half the value when the digits are multiplied. You can notice it if your screen is very large and you look closely. For mall screens , such minor reductions in resolution wont be noticeable. Try 640*480 , then you will realize what is changing.