These teacher-pupil ratios are for each class, not the ratio of teacher to total number of students that teacher has to deal with.
If I teach 6 hours per day and have 20 students in each one hour class, then I have 120 total students to deal with. If I teach 3 hours per day and have 20 students in each one hour class, then I have 60 total students to deal with. The teacher-pupil ratio is the same, but one teacher has double the teaching load of the other.
It is mathematically inescapable that if we have US teachers roughly cut in half the total number of teaching hours per year from roughly 1100 to be in line with the teachers of these other countries like Japan (600), Korea (550), Finland (600), and France (620), and if the total number of hours US students spend in front of a teacher per year are not cut, then we have to roughly double the number of US teachers.
The title of this thread is correct: US teachers are overworked and underpaid.