I get the feeling they don’t understand that they’re only about 16% to 18% of the population
They don’t. They live in echo chambers. They often live in states/cities/neighborhoods and spend time on websites full of people like themselves, and even if they don’t, they’ll naturally gravitate towards like minded people and befriend them. Many are also terminally online, and on the internet you can (usually) just block people you don’t want to listen to. So they surround themselves with like-minded people and rarely encounter opposing viewpoints that aren’t dumbed down, misinterpreted, or otherwise made to seem too ridiculous for any normal and reasonable person to believe them.
There is also a tendency (on all sides) to latch onto political candidates and assume that all of their supporters are a homogenous blob. Eg: if a left wing candidate wins an election with 54% of the vote, to assume that ALL of those voters are progressive activists, rather than only half or a third of them, with the rest being split between other constituencies that have different but somewhat compatible interests. But many people just look at election results and assume that half the population are just like them and the other half are the worst caricatures of the other side that they can imagine.
that wide portions of the population disagree with their moral framework for justified and logical reasons that don’t involve them all being hateful and/or Nazis?
No, they usually dismiss all opposition as being born out of ignorance, malice, or both. And many of the more radical ones lack a theory of mind.