Only those who got Es and higher on OWLs could continue for NEWTs, so NEWT students would be fewer than half. In Harry's year out of 40 students 25 (or maybe 26 if we include Hannah who left early in the year) qualified for the NEWT lessons in 6th year. And you still have to explain the number of Thestral-drawn carriages, which matches the higher number of students. (Or heck, how could there be 13 professional Quidditch teams.)