They may have been even taller when described in the book. It explained why they were tall, which was really interesting. The gravity on the planet was always pulling upwards, so everything on the planet was unnaturally tall and lanky.
Um, I'm not trying to be "that guy", but it's that Mars's gravity is a good deal less strong than Earth's. Kind of like how astronauts in full gear can leap around on the Moon (0.6 G) while they can only trudge on Earth (1 G). So the Seroni could get that tall without running afoul of the square-cube law. Lewis did his research: he also notes how cliffs and hills on Malacandra are steeper than on Earth, and how waves on a lake look extra-pointy, for the same reason.
Mar's gravity would be pulling up, as in not pulling down as much as Earth's does.
Man, it's details like that that make Lewis' worlds so carefully crafted and real, he was such an outstanding world builder and writer. It fills me with disbelief that these books were written when they were, they were so incredibly modern.