And at 0 stars and above, each upgrade of a star exponentially increases the soul bar, making it naturally less easy to hack away.
Another difference is between the souls of magic system and warrior system: the souls of mages obviously exceed those of the warrior system!
A player who equally upgrades both mage and warrior will have a soul bar roughly equivalent to their real life, like those omnipotent classes of the Order system, which have as much soul as they do real life. The difficulty of hacking at the soul bar is the same as hacking at the life bar.
On the other hand, if leaning towards Divine Arts damage, like with Light or Water elements which involve faith, the number of souls will be significantly higher than real life.