Arg .. ok, another problem detected and finally understood (another thanks to Xlat .. heh) :
16 bits samples in S3M have their size /2 stored, instead of the real size.
So, what changed is that the display remains the same ('sample' size), the total size calculation however uses this 'real' size.
BTW, this 16 bits feature in S3M is _not_ native. It's another improvement made by Impulse Tracker, no doubt, hence the lack of documentation on these additions to the format.