[HELP] Question about SSIM and PSNR results

[shazzla]

Hi !

I did some SSIM tests using FFMPEG,and something is not clear now.

The movies/clips are encoded using Hybrid/x265.
SSIM results are :

0.97xxxx84  (AQ Mode 3 ,size 35 MB)

0.97xxxx15 (AQ Mode 4 ,size 20 MB)

Both are visually the same.
But why is the big difference in size and only 0.00xxxx69 in score?
Who can explain this?

Btw i know about different quality measurments.

Oh,before i forget ,another question:
What is a good value in PSNR if the movie/clip is a 1080p encode?

Thanks in advance!