Aside from the airlines that fly Y-only configurations, even with full-service carriers the premium seat % is much smaller than 20% in short-haul ops.
And seats do not equal travellers. You can fill 20% of seats on planes with a lot smaller percentage of overall travellers. The premium seats tend to be filled with frequent flyers due to the logic of upgrades and awards (as well as the lifestyles of people who can afford F and J out of pocket), infrequent flyers tend to go with economy. So say 20,000 filled premium seats a year on a route may be occupied by just 5,000 different people whereas the 150,000 coach seats on the same route might be filled with something like 100,000 different people (just a calculation example).