I think you can't generalize. The '--tune xx' means specific encoder tweaks for optimizing certain criteria. '--tune ssim' is just one of these aiming at achieving the highest ssim metric. Better metrics do however not necessarily mean better visual experience.
'--tune xxx' and psy settings often increase the file size for a given CFR. So when disabled or overruled (e.g. by --tune ssim) the file size will become smaller. It also depends on the source.
Edit:
See the discussion here
https://superuser.com/questions/5644...n-of-x264-tune