well these comments seem useful. I will try this workaround suggested by erikl.
However, I got a good workaround "offline" from my senior colleague, and here I post it in case anybody face this again in future;
basically with Spectre simulator under virtuoso it seems not good not to save any data, because then the simulator will save a lot of -useless- data.
very simply, the solution was to pick a voltage node which is not or very rarely changing, gnd for instance, and select to save only this node.
Then I noticed the tran.dat file has a way smaller size than when no node was saved (from ~30 GB down to ~500MB).
and I may try also erikl's suggestions that probably could exclude the whole file!