/var/lib/mysql grows indefinitely in size

Having the need to upgrade from Ubuntu Bionic to Ubuntu Focal, I made an upgrade from server 5.7 - deduced afterward - to 8.0 by simply inheriting the original /var/lib/mysql directory, that is I left those files where they were (after making a backup of that directory). To get rid of some incompatibility found in the original configuration file (my.cnf) I purged everything from the old package and started the new 8 server with the configuration shipped with its package. Apparently, the server was working fine, that is I didn't find any particular complaint in /var/log/mysql/error.log and websites were finding their expected data. But after a few days, I found the disk space was totally or almost exhausted. I freed some space and noticed /var/lib/mysql has grown from about 38G to more than 100G (I issued the command du -sh * in /var/lib), and was still growing. What did I do wrong? Perhaps I made a big mistake in upgrading the server in the way I mentioned.