I really hate this pitting of state and independent teachers against each other.
This lazy trope that independent teachers 'can't handle' teaching in state schools and think they're better than state school teachers does none of us any favours. We're all just doing our best to teach the kids in front of us. All kids need and deserve a good education and kids don't choose which schools their parents send them to.
For what it's worth I'm an independent school teacher who started out in state before making the switch. As a secondary teacher I will say that independent school teaching is harder from the perspective of the more advanced academic level you are expected to teach to - sixth formers are really getting a first year university equivalent education in most good independent schools. There is little disruption in lessons so you have more time to teach and as a result are expected to get through more material to a higher level than in a state school. You'll find many teachers with PhDs and MA degrees in independent schools than you will state. This doesn't always translate into excellent classroom practice, of course, but it's a good indicator of how highly academically qualified independent teachers are expected to be, much more so than in state schools.
I would say that state school teaching is harder from a behaviour management perspective, and a classroom management perspective - getting around 32 kids in a lesson to check progress requires military planning that a teacher with only 18 kids doesn't have to do as rigorously.
I could go back and teach in a state school tomorrow quite easily - it would be more challenging in some ways but a lot easier in others. However, I don't want to, because in my independent school, I am paid more to work less. I have small class sizes, more free periods than teaching periods, and am paid a generous salary that affords me a very good quality of life. Why would I go back? Rather than railing at teachers who choose to teach in a workplace where they are valued and supported, you should direct your anger at the government. If schools were funded properly, state schools would be just as attractive places to work as independent schools. But they're not, so those of us with the ability to do so will jump ship. Teachers don't have to be martyrs to the cause, you know. We are allowed to look after our own best interests!
To the OP - a class size of 16-18 is perfect. I wouldn't want any less. A class size of 10 would become quite claustrophobic quite quickly!