These secondary Martians are bipedal, nearly  tall, and have "round, erect heads, and large eyes in flinty sockets"; however, their fragile physical structure, made up of weak skeletons and muscles, would have been broken by Earth's heavier gravitational pull.
These secondary Martians stand bipedally, approximately  tall, and possess "round, upright heads" with large eyes. Their fragile physical composition, including weak skeletons and muscles, would be adversely affected by Earth's heavier gravitational pull.