Hi, on my WSUS server, the DB disk is continuously filling up, even when the WSUS service is at 0% CPU use and no network trafic. Is there a reason ?

Generally, no. It depends on if you're performing the required maintenance, and what you have within your network and how many updates are approved. I've usually stayed below 150GB for the past 8 years in total usage. We have clients that have 300-500GB stores (down from 1.7TB), and clients that have less than 50GB in their data store. I'd say generally between 100-300GB is used, but all depending on what your network has and how you manage your WSUS server (approving updates - automatic vs approving only NEEDED updates like our new software AJ Tek Approvals does), along with what maintenance you're doing (if you're using WAM, you're covered there).