Both are listed by the US EPA as compact based on interior room, but the Mazda 3 has more interior room than the Toyota Corolla. The Mazda 6 is only slightly bigger in the passenger space than the Mazda 3, but has a more noticeable increase in trunk space (just enough to move into the US EPA midsize class).

Based on lists of Hertz cars by Hertz size class that I have seen, Hertz "intermediate" seems to range anywhere between small compacts (Toyota Corolla, Saturn Ion) to fairly large midsize cars or even full size cars (Kia Optima, Hyundai Sonata).

Several years ago, I rented from Hertz (in Canada). They had the Ford Focus as a compact, and the Chevrolet Cavalier as an intermediate. The Ford Focus has much more interior room than the Chevrolet Cavalier (plus, it is a better car), so why did they put the Chevrolet Cavalier in a higher price class?