Why are hardforks so much worse than softforks?

As I understand it, a softfork is a tightening of consensus rules, while a hardfork is a loosening of those rules.

Now, if we do a softfork and 10% don't follow the softfork, haven't those 10% effectively done a hardfork relative to the other 90%? (There is the saying that reversing a soft fork is a hard fork.)

Obviously, both people (the ones who followed the soft fork and the ones who didn't) are "compatible" with the old chain. However, the people who didn't follow the softfork may produce transactions, that are rejected by the nodes that followed the softfork.

So shouldn't the chain then split up? And if so, how is this less bad than a hardfork? I don't have to update my node for it to accept all transactions, but it may then produce invalid transactions as seen by the nodes that forked.

Or is it the miners that effectively decide?